diff --git a/crates/psd/src/file.rs b/crates/psd/src/file.rs
index 13c7a6a14..8404e38f5 100644
--- a/crates/psd/src/file.rs
+++ b/crates/psd/src/file.rs
@@ -488,7 +488,7 @@ impl PsdFile {
         let (v, widen) = self.write_form();
         let psb = v.is_psb();
         let len_field = |long: bool| if long { 8u64 } else { 4 };
-        let blocks_len = |blocks: &[TaggedBlock]| blocks.iter().fold(0u64, |n, b| n.saturating_add(b.encoded_len_aligned(v, 4) as u64));
+        let blocks_len = |blocks: &[TaggedBlock]| blocks.iter().fold(0u64, |n, b| n.saturating_add(b.encoded_len_aligned(v, 4)));
         let body_len = |info: &LayerInfo| -> Result<u64> {
             let unpadded = info.unpadded_len_as(v, widen)?;
             let padding = info.padding.as_ref().map_or(unpadded % 2, |p| p.len() as u64);
@@ -1051,9 +1051,10 @@ mod write_tests {
                         let mut materialized = file.clone();
                         let mut data = Vec::new();
                         materialized.layer_info.take().unwrap().write_body(&mut data, version).unwrap();
-                        materialized
-                            .global_blocks
-                            .insert(index.min(materialized.global_blocks.len()), TaggedBlock { signature: *b"8B64", key, data, padding });
+                        materialized.global_blocks.insert(
+                            index.min(materialized.global_blocks.len()),
+                            TaggedBlock { signature: *b"8B64", key, data, padding, stored: None },
+                        );
                         materialized.layer_info_placement = LayerInfoPlacement::Section;
                         assert_eq!(actual, materialized.to_bytes().unwrap(), "{version:?} {key:?} {index}");
                     }
diff --git a/crates/psd/src/layer/tests.rs b/crates/psd/src/layer/tests.rs
index 53cc409ed..423b017ae 100644
--- a/crates/psd/src/layer/tests.rs
+++ b/crates/psd/src/layer/tests.rs
@@ -322,8 +322,8 @@ fn unpadded_len_matches_the_written_body() {
             let mut rec = sample_record(version);
             rec.mask = MaskData::Mask(mask);
             rec.name = vec![b'n'; 300];
-            rec.blocks.push(TaggedBlock { signature: *b"8BIM", key: *b"lsct", data: vec![1; 5], padding: Some(vec![0; 3]) });
-            rec.blocks.push(TaggedBlock { signature: *b"8BIM", key: *b"Lr16", data: vec![2; 7], padding: None });
+            rec.blocks.push(TaggedBlock { signature: *b"8BIM", key: *b"lsct", data: vec![1; 5], padding: Some(vec![0; 3]), stored: None });
+            rec.blocks.push(TaggedBlock { signature: *b"8BIM", key: *b"Lr16", data: vec![2; 7], padding: None, stored: None });
             rec.extra_trailing = vec![9; 2];
             layers.push(rec);
         }
diff --git a/crates/psd/src/lazy.rs b/crates/psd/src/lazy.rs
index d05474ab9..271d22088 100644
--- a/crates/psd/src/lazy.rs
+++ b/crates/psd/src/lazy.rs
@@ -154,6 +154,31 @@ impl PsdFile {
         source: Source,
         want: impl FnOnce(&PsdFile, Option<Compression>, u64) -> Result<bool>,
         recover: impl FnOnce(&PsdFile) -> bool,
+    ) -> Result<(Self, Option<Result<Vec<u8>>>)> {
+        Self::from_source_deferring_info(source, |_, _| false, want, recover)
+    }
+
+    /// [`Self::from_source_and_merged_preflight`] that also leaves the data of each global block
+    /// `defer(key, length)` accepts in the source ([`TaggedBlock::stored`]): an embedded
+    /// smart-object block (`lnk2`) can be most of a file. Defer only blocks the reader loads
+    /// before use (the linked-file blocks `lnk2`/`lnk3`/`lnkD`): a deferred block's `data` is
+    /// empty, and [`TaggedBlock::parsed`] refuses it.
+    pub fn from_source_deferring(
+        source: Source,
+        defer: impl Fn(&[u8; 4], u64) -> bool,
+        want: impl FnOnce(&PsdFile) -> Result<bool>,
+        recover: impl FnOnce(&PsdFile) -> bool,
+    ) -> Result<(Self, Option<Result<Vec<u8>>>)> {
+        Self::from_source_deferring_info(source, defer, |file, _, _| want(file), recover)
+    }
+
+    /// [`Self::from_source_deferring`] whose preflight also sees the merged image's compression
+    /// and encoded length, as [`Self::from_source_and_merged_preflight_info`] gives them.
+    pub fn from_source_deferring_info(
+        source: Source,
+        defer: impl Fn(&[u8; 4], u64) -> bool,
+        want: impl FnOnce(&PsdFile, Option<Compression>, u64) -> Result<bool>,
+        recover: impl FnOnce(&PsdFile) -> bool,
     ) -> Result<(Self, Option<Result<Vec<u8>>>)> {
         let mut c = Cursor::new(&source);
         let header = Header::read(&mut Reader::new(&c.bytes(26)?))?;
@@ -221,6 +246,7 @@ impl PsdFile {
                         layer_info_placement = LayerInfoPlacement::GlobalBlock { index, signature, key, padding: padding.clone() };
                         Ok(Some(()))
                     },
+                    &defer,
                     &mut global_blocks,
                 )?;
                 layer_mask_trailing = trailing;
@@ -262,6 +288,7 @@ fn read_blocks<'a>(
     r: &mut Cursor<'a>,
     version: Version,
     mut consume: impl FnMut(usize, [u8; 4], [u8; 4], Cursor<'a>, &Option<Vec<u8>>) -> Result<Option<()>>,
+    defer: &dyn Fn(&[u8; 4], u64) -> bool,
     blocks: &mut Vec<TaggedBlock>,
 ) -> Result<Vec<u8>> {
     const ALIGNMENT: usize = 4;
@@ -291,8 +318,13 @@ fn read_blocks<'a>(
         let padding = if pad_bytes.len() == default && pad_bytes.iter().all(|&b| b == 0) { None } else { Some(pad_bytes) };
         let (start, end) = (data.pos, data.end);
         if consume(blocks.len(), signature, key, data, &padding)?.is_none() {
-            let data = Cursor { src: r.src, pos: start, end }.rest()?;
-            blocks.push(TaggedBlock { signature, key, data, padding });
+            if defer(&key, end - start) {
+                let stored = Stored { source: r.src.clone(), offset: start, len: end - start };
+                blocks.push(TaggedBlock { signature, key, data: Vec::new(), padding, stored: Some(stored) });
+            } else {
+                let data = Cursor { src: r.src, pos: start, end }.rest()?;
+                blocks.push(TaggedBlock { signature, key, data, padding, stored: None });
+            }
         }
     }
 }
@@ -436,6 +468,17 @@ mod tests {
         assert!(lazy.to_bytes().unwrap() == written, "the lazily read PSD writes another PSB");
     }
 
+    #[test]
+    fn deferred_blocks_read_back_the_same() {
+        for bytes in samples() {
+            let eager = PsdFile::from_bytes(&bytes).unwrap();
+            let (lazy, _) = PsdFile::from_source_deferring(Source::new(bytes.clone()), |_, _| true, |_| Ok(false), |_| false).unwrap();
+            assert!(lazy.global_blocks.iter().all(|b| b.data.is_empty() && b.stored.is_some()));
+            assert_eq!(lazy, eager);
+            assert_eq!(lazy.to_bytes().unwrap(), bytes);
+        }
+    }
+
     #[test]
     fn lazy_parse_fails_where_the_eager_one_does() {
         for bytes in samples() {
diff --git a/crates/psd/src/lib.rs b/crates/psd/src/lib.rs
index 42dc0b61a..2ad36b035 100644
--- a/crates/psd/src/lib.rs
+++ b/crates/psd/src/lib.rs
@@ -82,6 +82,7 @@ pub mod metadata;
 pub mod path;
 pub mod patterns;
 pub mod pixels;
+pub mod preview;
 pub mod resources;
 pub mod rows;
 pub mod shc;
@@ -109,6 +110,7 @@ pub use header::{ColorMode, Header, Version};
 pub use image_data::ImageData;
 pub use layer::{BlendingRanges, ChannelData, LayerFlags, LayerInfo, LayerMask, LayerRecord, MaskData, MaskParameters, RealMask, Rect};
 pub use pixels::{GrayImage, Layer, RgbaImage};
+pub use preview::Preview;
 pub use resources::{ImageResource, ResolutionInfo, ResourceData};
 pub use rows::RowDecoder;
 #[cfg(any(unix, windows))]
diff --git a/crates/psd/src/preview.rs b/crates/psd/src/preview.rs
new file mode 100644
index 000000000..04087b32c
--- /dev/null
+++ b/crates/psd/src/preview.rs
@@ -0,0 +1,158 @@
+//! A small picture of a file from its merged image, without decoding it whole:
+//! [`PsdFile::merged_preview`].
+//!
+//! Every `k`-th row of each plane is decoded (RLE and raw rows are read on their own, see
+//! [`crate::RowDecoder`]) and every `k`-th sample of it kept. For a file parsed with
+//! [`PsdFile::from_source`] that reads a few megabytes of a multi-gigabyte file, so the picture
+//! can show while the layers are still being decoded.
+
+use crate::compression::Compression;
+use crate::error::Result;
+use crate::file::PsdFile;
+use crate::header::ColorMode;
+
+/// A preview picture: RGBA, 8 bits per channel, row-major, opaque. A transparent document's
+/// merged image is stored matted with white, so the picture is the document over white.
+#[derive(Clone, Debug, PartialEq, Eq)]
+pub struct Preview {
+    /// Width in pixels.
+    pub width: u32,
+    /// Height in pixels.
+    pub height: u32,
+    /// `width × height × 4` bytes.
+    pub rgba: Vec<u8>,
+}
+
+impl PsdFile {
+    /// The merged image reduced (by a whole factor, nearest sample) to fit `max_side`. `None` when
+    /// there is no real merged image to show: an RGB or grayscale one (8/16/32-bit) stored raw
+    /// or RLE (a ZIP stream would have to be inflated whole), and not flagged as a placeholder.
+    pub fn merged_preview(&self, max_side: u32) -> Result<Option<Preview>> {
+        let h = &self.header;
+        let color = match h.color_mode {
+            ColorMode::Rgb => 3usize,
+            ColorMode::Grayscale => 1,
+            _ => return Ok(None),
+        };
+        if !matches!(h.depth, 8 | 16 | 32)
+            || !matches!(self.image_data.compression, Compression::Raw | Compression::Rle)
+            || self.has_real_merged_data() == Some(false)
+            || usize::from(h.channels) < color
+            || h.width == 0
+            || h.height == 0
+        {
+            return Ok(None);
+        }
+        let k = h.width.max(h.height).div_ceil(max_side.max(1)).max(1);
+        let (w, ht) = (h.width.div_ceil(k), h.height.div_ceil(k));
+        let bps = usize::from(h.depth / 8);
+        // At most `max_side`² pixels, but `max_side` is the caller's: checked and fallible.
+        let n = (w as usize)
+            .checked_mul(ht as usize)
+            .and_then(|p| p.checked_mul(4))
+            .filter(|&n| n as u64 <= crate::compression::MAX_DECODED_BYTES)
+            .ok_or(crate::error::PsdError::LimitExceeded("preview exceeds MAX_DECODED_BYTES"))?;
+        let dec = self.image_data.row_decoder(h)?;
+        let mut rgba = Vec::new();
+        rgba.try_reserve_exact(n).map_err(|_| crate::error::PsdError::OutOfMemory("not enough memory for the preview"))?;
+        rgba.resize(n, 255u8);
+        for y in 0..ht {
+            let src_y = (y * k) as usize;
+            for p in 0..color {
+                let r = p * h.height as usize + src_y;
+                let row = dec.rows(r, r + 1)?;
+                for x in 0..w as usize {
+                    let at = x * k as usize * bps;
+                    let Some(s) = row.get(at..at + bps) else { continue };
+                    let v = match *s {
+                        [a] => a,
+                        [a, _] => a,
+                        // 32-bit samples are linear light: encoded for display (sRGB) first.
+                        [a, b, c, d] => (srgb_encode(f32::from_be_bytes([a, b, c, d]).clamp(0.0, 1.0)) * 255.0).round() as u8,
+                        _ => 0,
+                    };
+                    let px = (y as usize * w as usize + x) * 4;
+                    if color == 1 {
+                        rgba[px..px + 3].fill(v);
+                    } else {
+                        rgba[px + p] = v;
+                    }
+                }
+            }
+        }
+        Ok(Some(Preview { width: w, height: ht, rgba }))
+    }
+}
+
+/// The sRGB transfer of a linear value in `0..=1`.
+fn srgb_encode(v: f32) -> f32 {
+    if v <= 0.003_130_8 { v * 12.92 } else { 1.055 * v.powf(1.0 / 2.4) - 0.055 }
+}
+
+#[cfg(test)]
+mod tests {
+    use crate::source::Source;
+    use crate::{Compression, PsdFile, Version, testgen};
+
+    /// The preview of a file read from a source is every k-th pixel of the merged image's
+    /// colour planes, opaque.
+    #[test]
+    fn preview_samples_the_merged_image() {
+        for c in [Compression::Raw, Compression::Rle] {
+            let file = testgen::small(Version::Psd, c);
+            let bytes = file.to_bytes().unwrap();
+            let lazy = PsdFile::from_source(Source::new(bytes)).unwrap();
+            let planes = file.decode_merged().unwrap();
+            let (w, h) = (file.header.width, file.header.height);
+            for max in [2, 3, 1000] {
+                let p = lazy.merged_preview(max).unwrap().unwrap();
+                let k = w.max(h).div_ceil(max);
+                assert_eq!((p.width, p.height), (w.div_ceil(k), h.div_ceil(k)));
+                for y in 0..p.height {
+                    for x in 0..p.width {
+                        let b = ((y * p.width + x) * 4) as usize;
+                        for ch in 0..3u32 {
+                            let a = (ch * w * h + y * k * w + x * k) as usize;
+                            assert_eq!(p.rgba[b + ch as usize], planes[a], "{c:?} {max} at {x},{y}");
+                        }
+                        assert_eq!(p.rgba[b + 3], 255);
+                    }
+                }
+            }
+        }
+        assert!(testgen::small(Version::Psd, Compression::Zip).merged_preview(4).unwrap().is_none());
+    }
+
+    /// The preview size is the caller's: one over the decode limit is refused before allocating.
+    #[test]
+    fn a_preview_over_the_decode_limit_is_refused() {
+        let mut file = testgen::small(Version::Psb, Compression::Raw);
+        (file.header.width, file.header.height) = (300_000, 300_000);
+        assert!(matches!(file.merged_preview(300_000), Err(crate::error::PsdError::LimitExceeded(_))));
+    }
+
+    /// Grayscale is replicated to RGB; 16-bit keeps each sample's high byte; 32-bit linear
+    /// samples are sRGB-encoded (0.5 is 188).
+    #[test]
+    fn preview_converts_grayscale_and_deep_samples() {
+        use crate::header::{ColorMode, Header};
+        use crate::image_data::ImageData;
+        let make = |mode: ColorMode, depth: u16, channels: u16, plane: Vec<u8>| {
+            let mut file = testgen::small(Version::Psd, Compression::Raw);
+            file.layer_info = None;
+            file.global_blocks.clear();
+            file.header = Header { channels, width: 2, height: 1, depth, color_mode: mode, ..file.header.clone() };
+            let planes: Vec<u8> = (0..channels).flat_map(|_| plane.clone()).collect();
+            file.image_data = ImageData::encode(Compression::Raw, &planes, &file.header).unwrap();
+            let bytes = file.to_bytes().unwrap();
+            PsdFile::from_source(Source::new(bytes)).unwrap().merged_preview(16).unwrap().unwrap()
+        };
+        let gray = make(ColorMode::Grayscale, 8, 1, vec![10, 200]);
+        assert_eq!(gray.rgba, vec![10, 10, 10, 255, 200, 200, 200, 255]);
+        let deep = make(ColorMode::Rgb, 16, 3, vec![0x12, 0x34, 0xAB, 0xCD]);
+        assert_eq!(deep.rgba, vec![0x12, 0x12, 0x12, 255, 0xAB, 0xAB, 0xAB, 255]);
+        let float: Vec<u8> = [0.5f32, 1.0].iter().flat_map(|v| v.to_be_bytes()).collect();
+        let f = make(ColorMode::Rgb, 32, 3, float);
+        assert_eq!(f.rgba, vec![188, 188, 188, 255, 255, 255, 255, 255]);
+    }
+}
diff --git a/crates/psd/src/rows.rs b/crates/psd/src/rows.rs
index 9fc95d36a..483785190 100644
--- a/crates/psd/src/rows.rs
+++ b/crates/psd/src/rows.rs
@@ -76,26 +76,54 @@ impl ChannelData {
     /// RLE count table, the whole of a ZIP stream); a damaged row fails when its band is decoded.
     pub fn row_decoder(&self, width: usize, height: usize, depth: u16, version: Version) -> Result<RowDecoder<'_>> {
         let layout = PlaneLayout { planes: 1, width, height, depth, version };
-        let total = layout.decoded_len()?;
-        let row_bytes = layout.row_bytes();
         let bytes = match &self.stored {
             Some(s) => Bytes::Source(s),
             None => Bytes::Memory(&self.data),
         };
-        let kind = match self.compression {
+        RowDecoder::new(self.compression, bytes, &layout, || self.encoded())
+    }
+}
+
+impl crate::image_data::ImageData {
+    /// A decoder of the merged image's rows: its planes one after another, so plane `p`'s row
+    /// `r` is row `p × height + r`. As [`ChannelData::row_decoder`]; only a ZIP merged image is
+    /// decoded whole (and so only one within `MAX_DECODED_BYTES`).
+    pub fn row_decoder(&self, header: &crate::header::Header) -> Result<RowDecoder<'_>> {
+        let layout = Self::layout(header);
+        let bytes = match &self.stored {
+            Some(s) => Bytes::Source(s),
+            None => Bytes::Memory(&self.data),
+        };
+        RowDecoder::new(Some(self.compression), bytes, &layout, || self.encoded())
+    }
+}
+
+impl<'a> RowDecoder<'a> {
+    fn new(
+        compression: Option<Compression>,
+        bytes: Bytes<'a>,
+        layout: &PlaneLayout,
+        encoded: impl FnOnce() -> Result<Cow<'a, [u8]>>,
+    ) -> Result<Self> {
+        // The whole-plane cap applies to a whole decode (ZIP) only: rows are decoded a band at a
+        // time, each band within the cap.
+        let total = layout.total_bytes()?;
+        let row_bytes = layout.row_bytes();
+        let height = layout.rows()?;
+        let kind = match compression {
             None if total == 0 => Kind::Empty,
             None => return Err(PsdError::invalid("channel has no data")),
             Some(Compression::Raw) => {
-                if bytes.len() < total as u64 {
+                if bytes.len() < total {
                     return Err(PsdError::UnexpectedEof {
                         offset: usize::try_from(bytes.len()).unwrap_or(usize::MAX),
-                        needed: total - bytes.len() as usize,
+                        needed: usize::try_from(total - bytes.len()).unwrap_or(usize::MAX),
                     });
                 }
                 Kind::Raw(bytes)
             }
             Some(Compression::Rle) => {
-                let size: u64 = if version.is_psb() { 4 } else { 2 };
+                let size: u64 = if layout.version.is_psb() { 4 } else { 2 };
                 let table_len = (height as u64).saturating_mul(size);
                 if table_len > bytes.len() {
                     return Err(PsdError::UnexpectedEof { offset: 0, needed: usize::try_from(table_len - bytes.len()).unwrap_or(usize::MAX) });
@@ -116,7 +144,7 @@ impl ChannelData {
                 }
                 let data_len = bytes.len() - table_len;
                 // As `decode_planes` checks: PackBits expands at most 64x.
-                if (total as u64) > data_len.saturating_mul(64) {
+                if total > data_len.saturating_mul(64) {
                     return Err(PsdError::Decompress("RLE data too short for declared size".into()));
                 }
                 if at > bytes.len() {
@@ -127,7 +155,7 @@ impl ChannelData {
                 }
                 Kind::Rle { bytes, offsets }
             }
-            Some(c) => Kind::Decoded(decode_planes(c, &self.encoded()?, &layout)?),
+            Some(c) => Kind::Decoded(decode_planes(c, &encoded()?, layout)?),
         };
         Ok(RowDecoder { kind, row_bytes, height })
     }
@@ -157,7 +185,7 @@ impl RowDecoder<'_> {
         out.try_reserve_exact(n).map_err(|_| PsdError::OutOfMemory("not enough memory for the decoded channel data"))?;
         match &self.kind {
             Kind::Empty => {}
-            Kind::Raw(bytes) => out.extend_from_slice(&bytes.get((r0 * self.row_bytes) as u64, n as u64)?),
+            Kind::Raw(bytes) => out.extend_from_slice(&bytes.get(r0 as u64 * self.row_bytes as u64, n as u64)?),
             Kind::Decoded(plane) => out
                 .extend_from_slice(plane.get(r0 * self.row_bytes..r1 * self.row_bytes).ok_or_else(|| PsdError::invalid("decoded plane too short"))?),
             Kind::Rle { bytes, offsets } => {
diff --git a/crates/psd/src/smart.rs b/crates/psd/src/smart.rs
index 64f5a23ca..7f05010f6 100644
--- a/crates/psd/src/smart.rs
+++ b/crates/psd/src/smart.rs
@@ -16,6 +16,7 @@
 //! The blocks themselves stay raw in the model ([`crate::TaggedBlock`]) and write back
 //! unchanged; these are read-only views. Came from EffectCraft's `effectcraft-psd`.
 
+use std::borrow::Cow;
 use std::ops::Range;
 
 use crate::descriptor::{Descriptor, UnicodeString, Value};
@@ -23,7 +24,6 @@ use crate::error::{PsdError, Result};
 use crate::file::PsdFile;
 use crate::io::{Reader, read_pascal};
 use crate::layer::LayerRecord;
-use crate::tagged::TaggedBlock;
 
 /// The most pixels a baked smart object may have: 2^24 (256 MiB as float RGBA). The size comes
 /// from the file's transform, which is untrusted.
@@ -458,19 +458,29 @@ fn linked_file(item: &[u8], at: usize) -> Result<LinkedFile> {
 }
 
 impl PsdFile {
-    /// Every linked file of the document's linked layer blocks, each with the block that holds
-    /// it ([`LinkedFile::data`] is a range of that block's data).
-    pub fn linked_files(&self) -> Vec<(&TaggedBlock, LinkedFile)> {
-        self.global_blocks
-            .iter()
-            .filter(|b| is_linked_layer_key(&b.key))
-            .flat_map(|b| linked_files(&b.data).into_iter().map(move |f| (b, f)))
-            .collect()
+    /// Every linked file of the document's linked layer blocks, each with the index of the
+    /// global block that holds it ([`LinkedFile::data`] is a range of that block's data). A
+    /// block left in the source ([`crate::TaggedBlock::stored`]) is read for it.
+    pub fn linked_files(&self) -> Result<Vec<(usize, LinkedFile)>> {
+        let mut out = Vec::new();
+        for (i, b) in self.global_blocks.iter().enumerate().filter(|(_, b)| is_linked_layer_key(&b.key)) {
+            out.extend(linked_files(&b.encoded()?).into_iter().map(|f| (i, f)));
+        }
+        Ok(out)
     }
 
-    /// The embedded bytes of the linked file `uuid`, when the document embeds it.
-    pub fn linked_data(&self, uuid: &str) -> Option<&[u8]> {
-        self.linked_files().into_iter().find_map(|(b, f)| if f.uuid == uuid { b.data.get(f.data?) } else { None })
+    /// The embedded bytes of the linked file `uuid`, when the document embeds it: borrowed from
+    /// a block in memory, read from the source for a block left there.
+    pub fn linked_data(&self, uuid: &str) -> Result<Option<Cow<'_, [u8]>>> {
+        for b in self.global_blocks.iter().filter(|b| is_linked_layer_key(&b.key)) {
+            let data = b.encoded()?;
+            let Some(range) = linked_files(&data).into_iter().find(|f| f.uuid == uuid).and_then(|f| f.data) else { continue };
+            return Ok(match data {
+                Cow::Borrowed(d) => d.get(range).map(Cow::Borrowed),
+                Cow::Owned(d) => d.get(range).map(|e| Cow::Owned(e.to_vec())),
+            });
+        }
+        Ok(None)
     }
 }
 
@@ -479,6 +489,7 @@ mod tests {
     use super::*;
     use crate::descriptor::{Id, ObjectArray};
     use crate::io::{WriteExt, write_pascal};
+    use crate::tagged::TaggedBlock;
 
     fn so(quad: [[f64; 2]; 4], warp: Option<Warp>) -> SmartObject {
         SmartObject { uuid: "x".into(), quad, size: None, warp }
@@ -651,15 +662,27 @@ mod tests {
     #[test]
     fn a_document_finds_the_embedded_bytes_of_a_smart_object() {
         let mut file = crate::testgen::small(crate::Version::Psd, crate::Compression::Raw);
+        let first = file.global_blocks.len();
         file.global_blocks.push(TaggedBlock::new(*b"lnk2", linked_record(b"liFD", "uuid-1", "a.png", b"PNGf", b"PNGDATA")));
         file.global_blocks.push(TaggedBlock::new(*b"lnkE", linked_record(b"liFE", "uuid-2", "b.psd", b"8BPS", b"")));
-        assert_eq!(file.linked_files().len(), 2);
-        assert_eq!(file.linked_data("uuid-1"), Some(b"PNGDATA".as_slice()));
-        assert_eq!(file.linked_data("uuid-2"), None);
-        assert_eq!(file.linked_data("nope"), None);
+        let embedded = |f: &PsdFile, uuid: &str| f.linked_data(uuid).unwrap().map(Cow::into_owned);
+        assert_eq!(
+            file.linked_files().unwrap().iter().map(|(i, f)| (*i, f.uuid.as_str())).collect::<Vec<_>>(),
+            [(first, "uuid-1"), (first + 1, "uuid-2")]
+        );
+        assert_eq!(embedded(&file, "uuid-1").as_deref(), Some(b"PNGDATA".as_slice()));
+        assert_eq!(embedded(&file, "uuid-2"), None);
+        assert_eq!(embedded(&file, "nope"), None);
         // The blocks are untouched: the file still writes and reads back.
         let again = PsdFile::from_bytes(&file.to_bytes().unwrap()).unwrap();
-        assert_eq!(again.linked_data("uuid-1"), Some(b"PNGDATA".as_slice()));
+        assert_eq!(embedded(&again, "uuid-1").as_deref(), Some(b"PNGDATA".as_slice()));
+        // With the linked-file blocks left in the source, the same files and bytes are found.
+        let bytes = file.to_bytes().unwrap();
+        let (deferred, _) =
+            PsdFile::from_source_deferring(crate::Source::new(bytes), |key, _| is_linked_layer_key(key), |_| Ok(false), |_| false).unwrap();
+        assert!(deferred.global_blocks.iter().filter(|b| is_linked_layer_key(&b.key)).all(|b| b.data.is_empty() && b.stored.is_some()));
+        assert_eq!(deferred.linked_files().unwrap(), file.linked_files().unwrap());
+        assert_eq!(embedded(&deferred, "uuid-1").as_deref(), Some(b"PNGDATA".as_slice()));
     }
 
     #[test]
diff --git a/crates/psd/src/source.rs b/crates/psd/src/source.rs
index 84aac1e60..8ecb8bb45 100644
--- a/crates/psd/src/source.rs
+++ b/crates/psd/src/source.rs
@@ -84,6 +84,10 @@ const MAX_READ: usize = 64 << 20;
 pub struct FileSource {
     file: std::fs::File,
     len: u64,
+    /// Modification time when opened: a read after the file changed in place fails rather than
+    /// mixing old offsets with new bytes. (A save that replaces the file by renaming leaves this
+    /// open file, the old one, unchanged.)
+    modified: Option<std::time::SystemTime>,
 }
 
 #[cfg(any(unix, windows))]
@@ -95,8 +99,8 @@ impl FileSource {
 
     /// Wraps an open file.
     pub fn new(file: std::fs::File) -> std::io::Result<Self> {
-        let len = file.metadata()?.len();
-        Ok(FileSource { file, len })
+        let meta = file.metadata()?;
+        Ok(FileSource { file, len: meta.len(), modified: meta.modified().ok() })
     }
 
     /// The file.
@@ -117,6 +121,10 @@ impl ReadAt for FileSource {
         }
         let n = buf.len();
         let failed = |e: std::io::Error| PsdError::Read(format!("reading {n} bytes at offset {offset}: {e}"));
+        let meta = self.file.metadata().map_err(failed)?;
+        if meta.len() != self.len || meta.modified().ok() != self.modified {
+            return Err(PsdError::Read("the file changed after it was opened".into()));
+        }
         // Bounded reads, in chunks: some Windows file
         // systems reject a single read of gigabytes (photocraft#375).
         let mut done = 0usize;
@@ -206,6 +214,11 @@ pub struct Stored {
 }
 
 impl Stored {
+    /// Whether `other` is the same bytes of the same source (a clone of it).
+    pub fn same_range(&self, other: &Stored) -> bool {
+        std::ptr::addr_eq(Arc::as_ptr(&self.source.0), Arc::as_ptr(&other.source.0)) && self.offset == other.offset && self.len == other.len
+    }
+
     /// Reads the bytes.
     pub fn read(&self) -> Result<Vec<u8>> {
         self.source.read_vec(self.offset, self.len)
@@ -265,6 +278,9 @@ mod tests {
         f.read_at(2, &mut b).unwrap();
         assert_eq!(b, [7, 6, 5]);
         assert!(f.read_at(3, &mut b).is_err());
+        // Changed in place after opening: reads fail instead of returning other bytes.
+        std::fs::OpenOptions::new().append(true).open(&path).unwrap().write_all(&[1]).unwrap();
+        assert!(f.read_at(0, &mut b).is_err());
         std::fs::remove_dir_all(&dir).unwrap();
     }
 }
diff --git a/crates/psd/src/tagged.rs b/crates/psd/src/tagged.rs
index 99ff3f91f..cc8b429ce 100644
--- a/crates/psd/src/tagged.rs
+++ b/crates/psd/src/tagged.rs
@@ -28,7 +28,11 @@ pub fn uses_long_length(version: Version, key: &[u8; 4]) -> bool {
 }
 
 /// A tagged block, stored raw.
-#[derive(Debug, Clone, PartialEq, Eq)]
+///
+/// A file parsed by [`PsdFile::from_source_deferring`](crate::PsdFile::from_source_deferring) may
+/// leave a large block's data in the source (`stored`, with `data` empty); equality compares the
+/// bytes wherever they are.
+#[derive(Debug, Clone)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
 pub struct TaggedBlock {
     /// `8BIM` or `8B64`.
@@ -42,8 +46,36 @@ pub struct TaggedBlock {
     /// layer blocks. Parsing yields `None` whenever
     /// the stored padding equals the default, so the model stays canonical.
     pub padding: Option<Vec<u8>>,
+    /// Where the data is when it was left in the source. (Not serialized: load it first.)
+    #[cfg_attr(feature = "serde", serde(skip))]
+    pub stored: Option<crate::source::Stored>,
 }
 
+impl PartialEq for TaggedBlock {
+    fn eq(&self, other: &Self) -> bool {
+        if self.signature != other.signature
+            || self.key != other.key
+            || self.padding != other.padding
+            || self.encoded_data_len() != other.encoded_data_len()
+        {
+            return false;
+        }
+        // The same bytes of the same source (a clone): equal without reading them, even when
+        // the source can no longer be read.
+        if let (Some(a), Some(b)) = (&self.stored, &other.stored)
+            && a.same_range(b)
+        {
+            return true;
+        }
+        match (self.encoded(), other.encoded()) {
+            (Ok(a), Ok(b)) => a == b,
+            _ => false,
+        }
+    }
+}
+
+impl Eq for TaggedBlock {}
+
 /// Layer section divider type (`lsct`).
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
@@ -138,7 +170,7 @@ pub enum BlockData<'a> {
 impl TaggedBlock {
     /// A new `8BIM` block with default padding.
     pub fn new(key: [u8; 4], data: Vec<u8>) -> Self {
-        TaggedBlock { signature: *b"8BIM", key, data, padding: None }
+        TaggedBlock { signature: *b"8BIM", key, data, padding: None, stored: None }
     }
 
     /// `true` for `MIB8` / `46B8`: the signatures [`crate::tiff`] gives a block whose data was
@@ -175,6 +207,11 @@ impl TaggedBlock {
     /// Typed view for known keys. `None` for keys without a typed parser,
     /// `Some(Err)` if a known key's data is malformed.
     pub fn parsed(&self) -> Option<Result<BlockData<'_>>> {
+        // Its views borrow `data`: a block still in the source must be loaded first, not read
+        // as empty.
+        if self.stored.is_some() {
+            return Some(Err(PsdError::invalid(format!("{}: block data is still in the source; load it first", self.key_str()))));
+        }
         let d = &self.data[..];
         let mut r = Reader::new(d);
         Some(match &self.key {
@@ -282,7 +319,9 @@ impl TaggedBlock {
     /// ("Additional Layer Information": Placed Layer, Placed Layer Data,
     /// Linked Layer, Object Based Effects Layer Info).
     pub fn check_structure(&self) -> Result<()> {
-        let d = &self.data[..];
+        // A block left in the source (an embedded smart-object file) is checked from there.
+        let data = self.encoded()?;
+        let d = &data[..];
         let mut r = Reader::new(d);
         match &self.key {
             b"PlLd" => {
@@ -333,7 +372,26 @@ impl TaggedBlock {
     }
 
     pub(crate) fn write(&self, out: &mut Vec<u8>, version: Version, alignment: usize) -> Result<()> {
-        Self::write_parts(out, self.signature, self.key, &self.data, self.padding.as_deref(), version, alignment)
+        Self::write_parts(out, self.signature, self.key, &self.encoded()?, self.padding.as_deref(), version, alignment)
+    }
+
+    /// The data: borrowed from `data`, or read from the source when it was left there.
+    pub fn encoded(&self) -> Result<std::borrow::Cow<'_, [u8]>> {
+        crate::source::encoded(&self.data, self.stored.as_ref())
+    }
+
+    /// Length of the data, wherever it is.
+    pub fn encoded_data_len(&self) -> u64 {
+        self.stored.as_ref().map_or(self.data.len() as u64, |s| s.len)
+    }
+
+    /// Reads data left in the source into `data`.
+    pub fn load(&mut self) -> Result<()> {
+        if let Some(s) = &self.stored {
+            self.data = s.read()?;
+            self.stored = None;
+        }
+        Ok(())
     }
 
     /// Writes a block from its parts (no owned [`TaggedBlock`] needed). `padding: None` writes
@@ -360,14 +418,16 @@ impl TaggedBlock {
 
     /// Serialized size in bytes as a layer block (two-byte alignment).
     pub fn encoded_len(&self, version: Version) -> usize {
-        self.encoded_len_aligned(version, 2)
+        usize::try_from(self.encoded_len_aligned(version, 2)).unwrap_or(usize::MAX)
     }
 
-    /// Serialized size in bytes as [`Self::write`] writes it with `alignment`.
-    pub(crate) fn encoded_len_aligned(&self, version: Version, alignment: usize) -> usize {
-        let l = if uses_long_length(version, &self.key) { 8 } else { 4 };
-        let p = self.padding.as_ref().map_or(Self::default_padding(self.data.len(), alignment), Vec::len);
-        (8 + l + p).saturating_add(self.data.len())
+    /// Serialized size in bytes as [`Self::write`] writes it with `alignment`. In `u64`: a
+    /// deferred PSB block can be longer than a 32-bit `usize`.
+    pub(crate) fn encoded_len_aligned(&self, version: Version, alignment: usize) -> u64 {
+        let l: u64 = if uses_long_length(version, &self.key) { 8 } else { 4 };
+        let n = self.encoded_data_len();
+        let p = self.padding.as_ref().map_or(Self::default_padding((n % alignment as u64) as usize, alignment), Vec::len);
+        (8 + l + p as u64).saturating_add(n)
     }
 }
 
@@ -499,7 +559,7 @@ pub(crate) fn read_blocks_with(
             Some(pad_bytes)
         };
         if !consume(blocks.len(), signature, key, data, &padding) {
-            blocks.push(TaggedBlock { signature, key, data: data.to_vec(), padding });
+            blocks.push(TaggedBlock { signature, key, data: data.to_vec(), padding, stored: None });
         }
     }
 }
@@ -523,12 +583,14 @@ pub(crate) fn write_blocks_to<W: std::io::Write + std::io::Seek + ?Sized>(
         out.small(|o| {
             o.put(&b.signature);
             o.put(&b.key);
-            o.put_len(b.data.len() as u64, uses_long_length(version, &b.key))
+            o.put_len(b.encoded_data_len(), uses_long_length(version, &b.key))
         })?;
-        out.put(&b.data)?;
+        // A block left in the source is copied from it piece by piece.
+        out.put_encoded(&b.data, b.stored.as_ref())?;
         match &b.padding {
             Some(p) => out.put(p)?,
-            None => out.zeros(TaggedBlock::default_padding(b.data.len(), alignment))?,
+            // From the length's remainder: a PSB block can be longer than a 32-bit `usize`.
+            None => out.zeros(TaggedBlock::default_padding((b.encoded_data_len() % alignment as u64) as usize, alignment))?,
         }
     }
     Ok(())
diff --git a/crates/psd/src/tiff.rs b/crates/psd/src/tiff.rs
index f18b29043..73a7caed3 100644
--- a/crates/psd/src/tiff.rs
+++ b/crates/psd/src/tiff.rs
@@ -185,7 +185,7 @@ impl ImageSourceData {
             let pad = TaggedBlock::default_padding(m.data.len(), 4);
             len = len.saturating_add(8 + len_field(b"LMsk")).saturating_add(m.data.len() as u64).saturating_add(pad as u64);
         }
-        len = self.global_blocks.iter().fold(len, |n, b| n.saturating_add(b.encoded_len_aligned(version, 4) as u64));
+        len = self.global_blocks.iter().fold(len, |n, b| n.saturating_add(b.encoded_len_aligned(version, 4)));
         len = len.saturating_add(self.trailing.len() as u64);
         let no_memory = || PsdError::OutOfMemory("not enough memory for the layer data");
         let mut body = Vec::new();
@@ -290,7 +290,7 @@ impl ImageSourceData {
 #[cfg(test)]
 fn padded_block(key: [u8; 4], data: Vec<u8>) -> TaggedBlock {
     let pad = (4 - data.len() % 4) % 4;
-    TaggedBlock { signature: *b"8BIM", key, data, padding: Some(vec![0; pad]) }
+    TaggedBlock { signature: *b"8BIM", key, data, padding: Some(vec![0; pad]), stored: None }
 }
 
 /// Parses a 34377 tag (image resources). Little-endian resource blocks (`MIB8`) are converted;
diff --git a/crates/psd/src/tiff/tests.rs b/crates/psd/src/tiff/tests.rs
index 6d8b5cdb4..1582da824 100644
--- a/crates/psd/src/tiff/tests.rs
+++ b/crates/psd/src/tiff/tests.rs
@@ -906,7 +906,7 @@ fn psb_foreign_order_long_key_roundtrips() {
     let d = ImageSourceData {
         version: Version::Psb,
         byte_order: ByteOrder::Little,
-        global_blocks: vec![TaggedBlock { signature: *b"MIB8", key: *b"Mt16", data: vec![9, 8, 7, 6, 5, 4], padding: None }],
+        global_blocks: vec![TaggedBlock { signature: *b"MIB8", key: *b"Mt16", data: vec![9, 8, 7, 6, 5, 4], padding: None, stored: None }],
         ..Default::default()
     };
     let (little, w) = d.to_bytes(ByteOrder::Little).unwrap();