diff --git a/src/libraries.rs b/src/libraries.rs
index 094c5ee..02d02ee 100644
--- a/src/libraries.rs
+++ b/src/libraries.rs
@@ -97,29 +97,118 @@ pub fn parse_excluded_dirs_column(raw: Option<&str>) -> Vec<String> {
     }
 }
 
-/// Canonicalise an excluded_dirs string for storage: parse → trim →
-/// dedupe (preserving insertion order) → comma-join with no inner
-/// whitespace. Empty / whitespace-only input → `None` (writes NULL).
+/// Validate a single excluded_dirs entry, normalising trivial cosmetic
+/// differences and rejecting forms that `PathExcluder` would silently
+/// drop. Returns the entry to store, or an error message describing
+/// what's wrong with it.
+///
+/// Rules:
+/// - Backslashes are rejected — PathExcluder strips only a leading `/`;
+///   a Windows-typed `\photos` or `photos\2024` lands in the
+///   component-pattern bucket and never matches anything. Suggest the
+///   forward-slash form.
+/// - A Windows drive letter prefix (`Z:` etc.) is rejected — excluded
+///   entries are *relative to the library root*, not absolute system
+///   paths.
+/// - A no-leading-slash entry containing `/` is rejected — the
+///   component-pattern path matches a single segment only; the user
+///   almost certainly meant the leading-slash form.
+/// - A `..` segment in a path entry is rejected — `base.join("../x")`
+///   doesn't canonicalise, so the resulting prefix never matches and
+///   the exclude silently fails.
+/// - Trailing slashes on path entries are stripped silently
+///   (`/photos/` → `/photos`) — purely cosmetic.
+pub fn validate_excluded_dirs_entry(entry: &str) -> Result<String, String> {
+    let trimmed = entry.trim();
+    if trimmed.is_empty() {
+        return Err("empty entry".to_string());
+    }
+    if trimmed.contains('\\') {
+        return Err(format!(
+            "'{}': use forward slashes — backslash paths never match on the watcher's component-by-component compare",
+            trimmed
+        ));
+    }
+    // Windows drive letter prefix like `Z:` or `Z:/something`. A
+    // length-2 ASCII-alpha + colon is the canonical form; we don't
+    // bother with longer multi-letter Windows drive-equivalents
+    // (`\\?\Volume{…}`) since the backslash check already catches them.
+    let bytes = trimmed.as_bytes();
+    if bytes.len() >= 2 && bytes[0].is_ascii_alphabetic() && bytes[1] == b':' {
+        return Err(format!(
+            "'{}': excluded entries are relative to the library root, not absolute system paths — drop the drive letter",
+            trimmed
+        ));
+    }
+    if let Some(rel) = trimmed.strip_prefix('/') {
+        // Path form. Reject `..` traversal — `base.join(\"../x\")` doesn't
+        // canonicalise, so `path.starts_with(...)` never matches.
+        if rel
+            .split('/')
+            .any(|seg| seg == "..")
+        {
+            return Err(format!(
+                "'{}': '..' segments don't normalise — the prefix-match never fires",
+                trimmed
+            ));
+        }
+        // Strip a trailing slash if any (`/photos/` → `/photos`). Purely
+        // cosmetic; PathBuf::starts_with treats both forms identically.
+        let stripped = if rel.ends_with('/') {
+            format!("/{}", rel.trim_end_matches('/'))
+        } else {
+            trimmed.to_string()
+        };
+        // After stripping, an empty rel ("/" alone) excludes the root —
+        // certainly a typo.
+        if stripped == "/" {
+            return Err("'/': excluding the library root is almost certainly a typo".to_string());
+        }
+        Ok(stripped)
+    } else {
+        // Component-pattern form: must be a single segment. A `/`
+        // anywhere here is the common "I forgot the leading slash" typo
+        // — reject so the user fixes it instead of staring at an
+        // exclude that does nothing.
+        if trimmed.contains('/') {
+            return Err(format!(
+                "'{}': multi-segment names only match with a leading slash — try '/{}'",
+                trimmed, trimmed
+            ));
+        }
+        Ok(trimmed.to_string())
+    }
+}
+
+/// Canonicalise an excluded_dirs string for storage: validate each
+/// entry, then parse → trim → dedupe (preserving insertion order) →
+/// comma-join with no inner whitespace. Empty / whitespace-only input
+/// → `Ok(None)` (writes NULL). Any entry that fails validation aborts
+/// the whole patch with a descriptive error so the operator can fix
+/// the typo before retrying.
 ///
 /// Used by `PATCH /libraries/{id}` so two users typing the same entries
 /// in different orders / casings / whitespace land on the same stored
 /// form, and a typo'd duplicate (`@eaDir, @eaDir`) collapses on save.
 /// Round-trip stable: writing the output back through this function
 /// yields the same string.
-pub fn normalize_excluded_dirs_input(raw: &str) -> Option<String> {
+pub fn normalize_excluded_dirs_input(raw: &str) -> Result<Option<String>, String> {
     let parsed = parse_excluded_dirs_column(Some(raw));
     if parsed.is_empty() {
-        return None;
+        return Ok(None);
     }
     let mut seen = std::collections::HashSet::new();
-    let deduped: Vec<String> = parsed
-        .into_iter()
-        .filter(|s| seen.insert(s.clone()))
-        .collect();
+    let mut deduped: Vec<String> = Vec::with_capacity(parsed.len());
+    for entry in parsed {
+        let validated = validate_excluded_dirs_entry(&entry)?;
+        if seen.insert(validated.clone()) {
+            deduped.push(validated);
+        }
+    }
     if deduped.is_empty() {
-        None
+        Ok(None)
     } else {
-        Some(deduped.join(","))
+        Ok(Some(deduped.join(",")))
     }
 }
 
@@ -454,11 +543,16 @@ pub async fn patch_library(
         }
     }
     if let Some(raw) = body.excluded_dirs.as_deref() {
-        // Canonicalise on write — trim, dedupe, drop empties — so the DB
-        // stores a round-trip-stable form regardless of how messy the
-        // user typed it. Empty / whitespace-only → NULL (matches a
-        // never-set library, parse_excluded_dirs_column returns []).
-        let normalised = normalize_excluded_dirs_input(raw);
+        // Canonicalise on write — trim, dedupe, validate, drop empties —
+        // so the DB stores a round-trip-stable form regardless of how
+        // messy the user typed it. Empty / whitespace-only → NULL
+        // (matches a never-set library). Validation failures (Windows
+        // backslash paths, drive letters, `..` traversal, etc.) bounce
+        // back as 422 so the operator can fix the typo.
+        let normalised = match normalize_excluded_dirs_input(raw) {
+            Ok(v) => v,
+            Err(msg) => return HttpResponse::UnprocessableEntity().body(msg),
+        };
         let stored: Option<&str> = normalised.as_deref();
         match diesel::update(libraries::table.filter(libraries::id.eq(lib_id)))
             .set(libraries::excluded_dirs.eq(stored))
@@ -693,10 +787,10 @@ mod tests {
 
     #[test]
     fn normalize_excluded_dirs_input_handles_empty_and_whitespace() {
-        assert_eq!(normalize_excluded_dirs_input(""), None);
-        assert_eq!(normalize_excluded_dirs_input("   "), None);
-        assert_eq!(normalize_excluded_dirs_input(",,,"), None);
-        assert_eq!(normalize_excluded_dirs_input(" , , "), None);
+        assert_eq!(normalize_excluded_dirs_input(""), Ok(None));
+        assert_eq!(normalize_excluded_dirs_input("   "), Ok(None));
+        assert_eq!(normalize_excluded_dirs_input(",,,"), Ok(None));
+        assert_eq!(normalize_excluded_dirs_input(" , , "), Ok(None));
     }
 
     #[test]
@@ -705,7 +799,7 @@ mod tests {
         // spaces. Mirrors how parse_excluded_dirs_column reads it back.
         assert_eq!(
             normalize_excluded_dirs_input(" @eaDir , /private , .thumbnails "),
-            Some("@eaDir,/private,.thumbnails".to_string())
+            Ok(Some("@eaDir,/private,.thumbnails".to_string()))
         );
     }
 
@@ -716,7 +810,7 @@ mod tests {
         // intent on round-trip).
         assert_eq!(
             normalize_excluded_dirs_input("@eaDir, /private, @eaDir, /private"),
-            Some("@eaDir,/private".to_string())
+            Ok(Some("@eaDir,/private".to_string()))
         );
         // Whitespace-distinct entries collapse to the same canonical
         // form. Case is preserved — `Foo` and `foo` are different keys
@@ -724,7 +818,7 @@ mod tests {
         // make that call here).
         assert_eq!(
             normalize_excluded_dirs_input(" Foo,foo, Foo "),
-            Some("Foo,foo".to_string())
+            Ok(Some("Foo,foo".to_string()))
         );
     }
 
@@ -734,8 +828,12 @@ mod tests {
         // same string. PATCH-clearing edits round-trip cleanly through
         // parse_excluded_dirs_column too.
         let raw = " /a/b ,, /a/b , c ";
-        let once = normalize_excluded_dirs_input(raw).expect("not empty");
-        let twice = normalize_excluded_dirs_input(&once).expect("not empty");
+        let once = normalize_excluded_dirs_input(raw)
+            .expect("validation passes")
+            .expect("not empty");
+        let twice = normalize_excluded_dirs_input(&once)
+            .expect("validation passes")
+            .expect("not empty");
         assert_eq!(once, twice);
         // Parsing the stored form back gives the deduped Vec.
         assert_eq!(
@@ -744,6 +842,102 @@ mod tests {
         );
     }
 
+    #[test]
+    fn validate_rejects_backslash_paths() {
+        // Windows-typed entries land in the component-pattern bucket
+        // and never match — reject so the user gets feedback instead
+        // of a silent no-op.
+        assert!(validate_excluded_dirs_entry(r"\photos").is_err());
+        assert!(validate_excluded_dirs_entry(r"photos\2024").is_err());
+        assert!(validate_excluded_dirs_entry(r"\\server\share").is_err());
+        // The error message names the entry and points at the fix.
+        let err = validate_excluded_dirs_entry(r"\photos").unwrap_err();
+        assert!(err.contains("forward slashes"), "{}", err);
+    }
+
+    #[test]
+    fn validate_rejects_windows_drive_letters() {
+        assert!(validate_excluded_dirs_entry("Z:/photos").is_err());
+        assert!(validate_excluded_dirs_entry("z:photos").is_err());
+        // Single-letter alpha + colon is the canonical drive prefix;
+        // the message should steer toward the relative form.
+        let err = validate_excluded_dirs_entry("Z:/foo").unwrap_err();
+        assert!(err.contains("relative to the library root"), "{}", err);
+    }
+
+    #[test]
+    fn validate_rejects_multi_segment_name_without_leading_slash() {
+        // The common "I forgot the slash" typo. Today this would store
+        // a never-matching component pattern; we catch it.
+        let err = validate_excluded_dirs_entry("photos/2024").unwrap_err();
+        assert!(err.contains("multi-segment"), "{}", err);
+        // And the suggestion shows the corrected form.
+        assert!(err.contains("/photos/2024"), "{}", err);
+    }
+
+    #[test]
+    fn validate_rejects_parent_dir_traversal_in_path_entries() {
+        // base.join("../sensitive") doesn't canonicalise, so the
+        // resulting prefix never starts_with anything the walker sees.
+        assert!(validate_excluded_dirs_entry("/../secret").is_err());
+        assert!(validate_excluded_dirs_entry("/photos/../keys").is_err());
+        // Same string as a non-leading-slash component is fine — it
+        // just never matches (you'd literally need a directory named
+        // `..` which is impossible on every filesystem we care about),
+        // but the validator accepts it because the failure mode isn't
+        // a silent footgun in that direction.
+        assert!(validate_excluded_dirs_entry("..").is_ok());
+    }
+
+    #[test]
+    fn validate_strips_trailing_slash_on_path_entries() {
+        assert_eq!(
+            validate_excluded_dirs_entry("/photos/").unwrap(),
+            "/photos"
+        );
+        assert_eq!(
+            validate_excluded_dirs_entry("/photos//").unwrap(),
+            "/photos"
+        );
+        // Bare "/" is rejected — almost certainly a typo for the
+        // library root.
+        assert!(validate_excluded_dirs_entry("/").is_err());
+        assert!(validate_excluded_dirs_entry("///").is_err());
+    }
+
+    #[test]
+    fn validate_passes_valid_entries() {
+        for entry in &[
+            "/photos",
+            "/photos/2024",
+            "/media/raw/private",
+            "@eaDir",
+            ".thumbnails",
+            ".DS_Store",
+            "node_modules",
+        ] {
+            assert!(
+                validate_excluded_dirs_entry(entry).is_ok(),
+                "expected {} to pass",
+                entry
+            );
+        }
+    }
+
+    #[test]
+    fn normalize_aborts_on_invalid_entry() {
+        // One bad entry kills the whole patch — better to surface the
+        // problem than to silently apply N-1 of N changes.
+        let err = normalize_excluded_dirs_input("/photos, photos/2024").unwrap_err();
+        assert!(err.contains("photos/2024"), "{}", err);
+        // A valid mix succeeds — the bad-entry test isn't accidentally
+        // matching the good prefix.
+        assert_eq!(
+            normalize_excluded_dirs_input("/photos, @eaDir, /private/"),
+            Ok(Some("/photos,@eaDir,/private".to_string()))
+        );
+    }
+
     fn probe_lib(id: i32, root: String) -> Library {
         Library {
             id,