diff --git a/crates/engine/src/cmd/previews.rs b/crates/engine/src/cmd/previews.rs
index a1715d79..ea2ccb00 100644
--- a/crates/engine/src/cmd/previews.rs
+++ b/crates/engine/src/cmd/previews.rs
@@ -194,10 +194,7 @@ fn smart_run(
         let _one = SMART_WRITE.lock().unwrap_or_else(std::sync::PoisonError::into_inner);
         // Read either the new shard or a pre-upgrade flat file, without making new work
         // depend on having the original online.
-        let path = if path.exists() { path } else {
-            let legacy = path.file_name().map(|name| dir.join(name));
-            legacy.filter(|old| old.exists()).unwrap_or(path)
-        };
+        let path = crate::smart::existing_file_path(dir, path);
         if discard {
             if std::fs::remove_file(&path).is_ok() {
                 n.removed += 1;
@@ -329,7 +326,7 @@ pub(crate) fn refresh_stale_smart_previews(s: &mut Session) -> Option<std::threa
     let max_edge = crate::media::SourceLevel::Preview.max_edge();
     let photos: Vec<_> = s.catalog.photos().filter(|p| matches!(p.source, lightcraft_catalog::Source::File { .. }) && p.relative_wb()).collect();
     let jobs: Vec<SmartJob> =
-        photos.iter().map(|p| (p.id, crate::smart::file_path(&dir, p), Some(s.media.origin_ref(&p.source, max_edge)), true)).collect();
+        photos.iter().map(|p| (p.id, crate::smart::sharded_file_path(&dir, p), Some(s.media.origin_ref(&p.source, max_edge)), true)).collect();
     std::thread::Builder::new()
         .name("lc-smart-look".into())
         .spawn(move || refresh_stale(&dir, jobs))
@@ -340,7 +337,11 @@ pub(crate) fn refresh_stale_smart_previews(s: &mut Session) -> Option<std::threa
 /// The body of [`refresh_stale_smart_previews`]: rebuild the stale proxies among `jobs`.
 #[cfg(not(target_arch = "wasm32"))]
 fn refresh_stale(dir: &std::path::Path, jobs: Vec<SmartJob>) {
-    let stale: Vec<SmartJob> = jobs.into_iter().filter(|(_, path, _, _)| crate::smart::is_stale(path)).collect();
+    let stale: Vec<SmartJob> = jobs
+        .into_iter()
+        .map(|(id, path, source, look)| (id, crate::smart::existing_file_path(dir, path), source, look))
+        .filter(|(_, path, _, _)| crate::smart::is_stale(path))
+        .collect();
     if !stale.is_empty() {
         // `custom`: the folder is there already, and must not be recreated if it went away since
         match smart_run(dir, true, false, stale, None) {
diff --git a/crates/engine/src/smart.rs b/crates/engine/src/smart.rs
index cd003be4..532454a1 100644
--- a/crates/engine/src/smart.rs
+++ b/crates/engine/src/smart.rs
@@ -31,9 +31,18 @@ pub fn sharded_file_path(dir: &Path, p: &Photo) -> PathBuf {
 /// Read an existing proxy in either layout. New proxies use the sharded location; previews
 /// made by older releases still load and can be moved with smartPreviewsLocation.
 pub fn file_path(dir: &Path, p: &Photo) -> PathBuf {
-    let sharded = sharded_file_path(dir, p);
-    let legacy = dir.join(file_name(p));
-    if sharded.exists() || !legacy.exists() { sharded } else { legacy }
+    existing_file_path(dir, sharded_file_path(dir, p))
+}
+
+/// Resolve a sharded path to an older flat file when present. Call only off the UI thread:
+/// checking a removable drive synchronously could stall drawing.
+pub fn existing_file_path(dir: &Path, sharded: PathBuf) -> PathBuf {
+    if sharded.exists() {
+        return sharded;
+    }
+    let Some(name) = sharded.file_name() else { return sharded };
+    let legacy = dir.join(name);
+    if legacy.exists() { legacy } else { sharded }
 }
 
 /// The most header [`is_valid`] reads (the JSON line before the JPEG).
@@ -189,7 +198,7 @@ pub fn migrate(from: &Path, to: &Path, what: Existing) -> (usize, Vec<String>) {
         return (done, failed);
     }
     for src in files(from) {
-        let name = src.file_name().unwrap_or_default();
+        let Some(name) = src.file_name() else { continue };
         let dst = name.to_str().and_then(|n| n.strip_suffix(".lcsp")).filter(|stem| {
             stem.len() == 32 && stem.bytes().all(|b| b.is_ascii_hexdigit() && !b.is_ascii_uppercase())
         }).map_or_else(|| to.join(name), |stem| to.join(&stem[..2]).join(name));
diff --git a/crates/engine/src/tests_import.rs b/crates/engine/src/tests_import.rs
index 193fdce8..7d257ffc 100644
--- a/crates/engine/src/tests_import.rs
+++ b/crates/engine/src/tests_import.rs
@@ -795,6 +795,13 @@ fn smart_previews_folder_is_chosen_per_library() {
     assert_eq!(loc["path"], lib.join("Smart Previews").to_string_lossy().as_ref());
     s.execute("library.smartPreviews", &json!({})).unwrap();
     assert_eq!(crate::smart::stats(&lib.join("Smart Previews")).0, 1);
+    // A flat proxy from older builds remains discoverable and migrates to the new sharded layout.
+    let dir = lib.join("Smart Previews");
+    let photo = s.catalog.photo(id).unwrap();
+    let shard = crate::smart::sharded_file_path(&dir, photo);
+    let flat = dir.join(crate::smart::file_name(photo));
+    std::fs::rename(&shard, &flat).unwrap();
+    assert_eq!(s.execute("photo.smartPreview", &json!({})).unwrap()["smartPreview"], true);
 
     // relative paths and missing drives are refused, and nothing is created on this drive
     assert!(s.execute("library.smartPreviewsLocation", &json!({"path": "rel/dir"})).is_err());