diff --git a/Cargo.lock b/Cargo.lock
index 298258ca..bb5d87f0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3978,6 +3978,7 @@ dependencies = [
  "png 0.18.1",
  "pollster 1.0.1",
  "rfd",
+ "rustix 1.1.5",
  "serde",
  "serde_json",
  "skrifa 0.44.0",
diff --git a/crates/ui-egui/Cargo.toml b/crates/ui-egui/Cargo.toml
index 8ee4e68e..5400829c 100644
--- a/crates/ui-egui/Cargo.toml
+++ b/crates/ui-egui/Cargo.toml
@@ -48,6 +48,9 @@ skrifa = "0.44"
 objc2 = "0.6"
 objc2-app-kit = { version = "0.3", default-features = false, features = ["std", "NSEvent", "NSScreen"] }
 objc2-foundation = { version = "0.3", default-features = false, features = ["std", "NSArray", "NSGeometry"] }
+# A plain fsync where a volume refuses the full flush `sync_all` asks for (SMB, FUSE); already in
+# the tree through pdfcraft-cli.
+rustix = { version = "1", features = ["fs"] }
 
 [target.'cfg(windows)'.dependencies]
 # Auto interface language: the Windows display language, without starting a process.
diff --git a/crates/ui-egui/src/editing.rs b/crates/ui-egui/src/editing.rs
index b3b02a38..9b1b35b1 100644
--- a/crates/ui-egui/src/editing.rs
+++ b/crates/ui-egui/src/editing.rs
@@ -923,6 +923,17 @@ pub fn write_atomically(path: &str, bytes: &[u8]) -> std::io::Result<()> {
 
 /// [`write_atomically`], trying the staging names that `suffixes` give.
 fn write_atomically_with(path: &str, bytes: &[u8], suffixes: impl IntoIterator<Item = u64>) -> std::io::Result<()> {
+    write_atomically_syncing(path, bytes, suffixes, std::fs::File::sync_all)
+}
+
+/// [`write_atomically_with`], flushing the staging file with `sync_all` (the standard library's
+/// in the app, a failing one in tests).
+fn write_atomically_syncing(
+    path: &str,
+    bytes: &[u8],
+    suffixes: impl IntoIterator<Item = u64>,
+    sync_all: impl FnOnce(&std::fs::File) -> std::io::Result<()>,
+) -> std::io::Result<()> {
     use std::io::Write;
     let target = std::path::Path::new(path);
     let dir = target.parent().filter(|d| !d.as_os_str().is_empty()).unwrap_or(std::path::Path::new("."));
@@ -933,7 +944,7 @@ fn write_atomically_with(path: &str, bytes: &[u8], suffixes: impl IntoIterator<I
         {
             let mut f = f;
             f.write_all(bytes)?;
-            f.sync_all()?;
+            sync_staged(&f, sync_all)?;
         }
         std::fs::rename(&tmp, target)
     })();
@@ -943,6 +954,23 @@ fn write_atomically_with(path: &str, bytes: &[u8], suffixes: impl IntoIterator<I
     result
 }
 
+/// Flush the staging file to the disk before it replaces the original. On macOS `sync_all` asks
+/// for a full flush (`F_FULLFSYNC`), which SMB and FUSE volumes, such as a mounted Cryptomator
+/// vault, refuse as unsupported: there a plain `fsync` is the strongest flush the volume offers.
+/// Any other failure still fails the save.
+fn sync_staged(f: &std::fs::File, sync_all: impl FnOnce(&std::fs::File) -> std::io::Result<()>) -> std::io::Result<()> {
+    #[cfg(target_os = "macos")]
+    {
+        use rustix::io::Errno;
+        match sync_all(f) {
+            Err(e) if matches!(Errno::from_io_error(&e), Some(Errno::NOTSUP | Errno::NOTTY)) => Ok(rustix::fs::fsync(f)?),
+            other => other,
+        }
+    }
+    #[cfg(not(target_os = "macos"))]
+    sync_all(f)
+}
+
 #[cfg(not(target_arch = "wasm32"))]
 fn short_name(path: &str) -> String {
     std::path::Path::new(path).file_name().map(|n| n.to_string_lossy().into_owned()).unwrap_or_else(|| path.to_string())
@@ -1028,7 +1056,7 @@ fn bookmark_at<'a>(items: &'a [pdfcraft_render::OutlineItem], path: &[usize]) ->
 
 #[cfg(test)]
 mod tests {
-    use super::{SaveTarget, is_mail_temp_copy, write_atomically, write_atomically_with};
+    use super::{SaveTarget, is_mail_temp_copy, write_atomically, write_atomically_syncing, write_atomically_with};
     use pdfcraft_platform::staging::{STAGING_ATTEMPTS, staging_suffixes};
     use std::path::{Path, PathBuf};
 
@@ -1185,6 +1213,43 @@ mod tests {
         std::fs::set_permissions(&target, perms).unwrap();
         let _ = std::fs::remove_dir_all(&dir);
     }
+
+    /// On macOS `sync_all` asks for a full flush, which SMB and FUSE volumes (a Cryptomator vault,
+    /// #981) refuse as unsupported: ENOTSUP (45) or ENOTTY (25). The save still goes through.
+    #[cfg(target_os = "macos")]
+    #[test]
+    fn a_volume_without_full_flush_still_saves() {
+        for errno in [45, 25] {
+            let dir = staging_dir(&format!("nofullsync-{errno}"));
+            let target = dir.join("out.pdf");
+            std::fs::write(&target, "OLD").unwrap();
+            let asked = std::cell::Cell::new(false);
+            let refuse = |_: &std::fs::File| {
+                asked.set(true);
+                Err(std::io::Error::from_raw_os_error(errno))
+            };
+            write_atomically_syncing(&target.to_string_lossy(), b"NEW", [7], refuse).unwrap();
+            assert!(asked.get(), "the full flush was tried");
+            assert_eq!(read(&target), "NEW");
+            assert_eq!(listing(&dir), ["out.pdf"], "no staging file is left behind");
+            let _ = std::fs::remove_dir_all(&dir);
+        }
+    }
+
+    /// Only "not supported" is let through: any other flush failure fails the save and keeps the
+    /// original.
+    #[test]
+    fn a_failed_flush_keeps_the_original() {
+        let dir = staging_dir("syncfails");
+        let target = dir.join("out.pdf");
+        std::fs::write(&target, "OLD").unwrap();
+        let e = write_atomically_syncing(&target.to_string_lossy(), b"NEW", [7], |_| Err(std::io::Error::other("disk gone"))).unwrap_err();
+        assert_eq!(e.to_string(), "disk gone");
+        assert_eq!(read(&target), "OLD");
+        assert_eq!(listing(&dir), ["out.pdf"], "the staging file was removed");
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
     #[test]
     fn outlook_attachment_copies_are_recognised_on_either_separator_and_in_any_case() {
         for p in [