diff --git a/apps/root.immich.app/src/lib/compose/config.test.ts b/apps/root.immich.app/src/lib/compose/config.test.ts
index f511e178..51529d7a 100644
--- a/apps/root.immich.app/src/lib/compose/config.test.ts
+++ b/apps/root.immich.app/src/lib/compose/config.test.ts
@@ -152,6 +152,42 @@ describe('validate', () => {
     }
   });
 
+  it('refuses an external library that lands inside the container upload directory', () => {
+    const config = structuredClone(DEFAULT_CONFIG);
+
+    for (const path of ['/data', '/data/library', '/data/upload', '/data/thumbs', '/data/anything']) {
+      config.storage.externalLibraries = [{ path, readOnly: true }];
+      expect(validate(config)['storage.externalLibraries.0.path'], path).toContain('/data');
+    }
+  });
+
+  it('refuses the filesystem root as an external library', () => {
+    const config = structuredClone(DEFAULT_CONFIG);
+
+    for (const path of ['/', '//']) {
+      config.storage.externalLibraries = [{ path, readOnly: true }];
+      expect(validate(config)['storage.externalLibraries.0.path'], path).toContain('root');
+    }
+  });
+
+  it('flags an external library that reuses an override host path', () => {
+    const config = structuredClone(DEFAULT_CONFIG);
+    config.storage.customFolders = true;
+    config.storage.overrides.thumbs = '/mnt/thumbs';
+    config.storage.externalLibraries = [{ path: '/mnt/thumbs', readOnly: true }];
+    const errors = validate(config);
+    expect(errors['storage.overrides.thumbs']).toBeTruthy();
+    expect(errors['storage.externalLibraries.0.path']).toBeTruthy();
+  });
+
+  it('still accepts an ordinary external library alongside overrides', () => {
+    const config = structuredClone(DEFAULT_CONFIG);
+    config.storage.customFolders = true;
+    config.storage.overrides.thumbs = '/mnt/fast/thumbs';
+    config.storage.externalLibraries = [{ path: '/mnt/media/photos', readOnly: true }];
+    expect(validate(config)).toEqual({});
+  });
+
   it('flags an external library that overlaps another mount', () => {
     const config = structuredClone(DEFAULT_CONFIG);
     config.storage.uploadLocation = '/mnt/media';
diff --git a/apps/root.immich.app/src/lib/compose/config.ts b/apps/root.immich.app/src/lib/compose/config.ts
index 685a5dfe..87fdfd9e 100644
--- a/apps/root.immich.app/src/lib/compose/config.ts
+++ b/apps/root.immich.app/src/lib/compose/config.ts
@@ -264,6 +264,8 @@ type ValidationErrors = Record<string, string>;
 
 const normalizePath = (path: string) => path.trim().replace(/\/+$/, '');
 
+const UPLOAD_TARGET = '/data';
+
 type Mount = { path: string[]; location: string };
 
 const collectMounts = (config: ImmichConfig): Mount[] => {
@@ -301,11 +303,18 @@ const validationSchema = immichConfigSchema.superRefine((config, ctx) => {
   }
 
   for (const [index, { path }] of config.storage.externalLibraries.entries()) {
-    const location = path.trim();
-    if (!location) {
-      fail(['storage', 'externalLibraries', String(index), 'path'], 'A path is required.');
+    const field = ['storage', 'externalLibraries', String(index), 'path'];
+    const trimmed = path.trim();
+    const location = normalizePath(path);
+
+    if (!trimmed) {
+      fail(field, 'A path is required.');
+    } else if (!location) {
+      fail(field, 'The filesystem root cannot be mounted as a library.');
     } else if (!location.startsWith('/')) {
-      fail(['storage', 'externalLibraries', String(index), 'path'], 'Enter an absolute path, e.g. /mnt/media/photos.');
+      fail(field, 'Enter an absolute path, e.g. /mnt/media/photos.');
+    } else if (location === UPLOAD_TARGET || location.startsWith(`${UPLOAD_TARGET}/`)) {
+      fail(field, `Immich manages ${UPLOAD_TARGET} inside the container, so a library cannot live there.`);
     }
   }