diff --git a/src/calibre/library/check_library.py b/src/calibre/library/check_library.py
index 65cfaaaa39..6b188afec2 100644
--- a/src/calibre/library/check_library.py
+++ b/src/calibre/library/check_library.py
@@ -34,12 +34,12 @@ class CheckLibrary(object):
         self.src_library_path = os.path.abspath(library_path)
         self.db = db
 
+        self.is_case_sensitive = db.is_case_sensitive
+
         self.all_authors = frozenset([x[1] for x in db.all_authors()])
         self.all_ids = frozenset([id for id in db.all_ids()])
-        self.is_case_sensitive = db.is_case_sensitive
         self.all_dbpaths = frozenset(self.dbpath(id) for id in self.all_ids)
-        self.all_lc_dbpaths = frozenset(self.dbpath(id).lower()
-                                                for id in self.all_ids)
+        self.all_lc_dbpaths = frozenset([f.lower() for f in self.all_dbpaths])
 
         self.db_id_regexp = re.compile(r'^.* \((\d+)\)$')
         self.bad_ext_pat = re.compile(r'[^a-z]+')
@@ -132,35 +132,50 @@ class CheckLibrary(object):
     def process_book(self, lib, book_info):
         (db_path, title_dir, book_id) = book_info
         filenames = frozenset(os.listdir(os.path.join(lib, db_path)))
-        filenames_lc = frozenset(f.lower() for f in filenames)
         book_id = int(book_id)
         formats = frozenset(filter(self.is_ebook_file, filenames))
-        formats_lc = frozenset(f.lower() for f in formats)
-
-        # Check: any books that aren't formats or normally there?
-        if self.is_case_sensitive:
-            unknowns = frozenset(filenames-formats-NORMALS)
-        else:
-            unknowns = frozenset(filenames_lc-formats_lc-NORMALS)
-        if unknowns:
-            self.extra_files.append((title_dir, db_path, unknowns))
-
         book_formats = frozenset([x[0]+'.'+x[1].lower() for x in
                             self.db.format_files(book_id, index_is_id=True)])
-        book_formats_lc = frozenset(f.lower() for f in book_formats)
 
-        # Check: any book formats that should be there?
         if self.is_case_sensitive:
+            unknowns = frozenset(filenames-formats-NORMALS)
+            # Check: any books that aren't formats or normally there?
+            if unknowns:
+                self.extra_files.append((title_dir, db_path, unknowns))
+
+            # Check: any book formats that should be there?
             missing = book_formats - formats
-        else:
-            missing = book_formats_lc - formats_lc
-        if missing:
-            self.missing_formats.append((title_dir, db_path, missing))
+            if missing:
+                self.missing_formats.append((title_dir, db_path, missing))
 
-        # Check: any book formats that shouldn't be there?
-        if self.is_case_sensitive:
+            # Check: any book formats that shouldn't be there?
             extra = formats - book_formats
+            if extra:
+                self.extra_formats.append((title_dir, db_path, extra))
         else:
+            def lc_map(fnames, fset):
+                m = {}
+                for f in fnames:
+                    m[f.lower()] = f
+                return [m[f] for f in fset]
+
+            filenames_lc = frozenset([f.lower() for f in filenames])
+            formats_lc = frozenset([f.lower() for f in formats])
+            unknowns = frozenset(filenames_lc-formats_lc-NORMALS)
+            # Check: any books that aren't formats or normally there?
+            if unknowns:
+                self.extra_files.append((title_dir, db_path,
+                                         lc_map(filenames, unknowns)))
+
+            book_formats_lc = frozenset([f.lower() for f in book_formats])
+            # Check: any book formats that should be there?
+            missing = book_formats_lc - formats_lc
+            if missing:
+                self.missing_formats.append((title_dir, db_path,
+                                             lc_map(book_formats, missing)))
+
+            # Check: any book formats that shouldn't be there?
             extra = formats_lc - book_formats_lc
-        if extra:
-            self.extra_formats.append((title_dir, db_path, extra))
+            if extra:
+                self.extra_formats.append((title_dir, db_path,
+                                           lc_map(formats, extra)))