diff --git a/datasette/app.py b/datasette/app.py
index 6b0e6ada..c0071d2a 100644
--- a/datasette/app.py
+++ b/datasette/app.py
@@ -186,7 +186,8 @@ class Datasette:
             if db.name in self.databases:
                 raise Exception("Multiple files with same stem: {}".format(db.name))
             self.add_database(db.name, db)
-        self.scan_dirs()
+        self.scan_dirs_executor = futures.ThreadPoolExecutor(max_workers=1)
+        self.scan_dirs_executor.submit(self.scan_dirs)
         self.cache_headers = cache_headers
         self.cors = cors
         self._metadata = metadata or {}
@@ -224,7 +225,6 @@ class Datasette:
 
     def scan_dirs(self):
         # Recurse through self.dirs looking for new SQLite DBs
-        i = 0
         for dir in self.dirs:
             print(dir)
             for filepath in Path(dir).glob("**/*.db"):
@@ -237,9 +237,6 @@ class Datasette:
                         .replace(".db", ""),
                         Database(self, filepath, is_mutable=True),
                     )
-                    i += 1
-                    if i >= 20:
-                        break
 
     def config(self, key):
         return self._config.get(key, None)
diff --git a/datasette/views/index.py b/datasette/views/index.py
index fe88a38c..91988d1c 100644
--- a/datasette/views/index.py
+++ b/datasette/views/index.py
@@ -23,7 +23,10 @@ class IndexView(BaseView):
 
     async def get(self, request, as_format):
         databases = []
-        for name, db in self.ds.databases.items():
+        # Using list() here because scan_dirs() running in a thread might
+        # modify self.ds.databases while we are iterating it, which could
+        # cause 'RuntimeError: OrderedDict mutated during iteration'
+        for name, db in list(self.ds.databases.items()):
             table_names = await db.table_names()
             hidden_table_names = set(await db.hidden_table_names())
             views = await db.view_names()