diff --git a/pycosio/_core/io_system.py b/pycosio/_core/io_system.py
index 123863b..0c30839 100644
--- a/pycosio/_core/io_system.py
+++ b/pycosio/_core/io_system.py
@@ -20,9 +20,10 @@ class SystemBase(ABC):
             Generally, client configuration and credentials.
         unsecure (bool): If True, disables TLS/SSL to improves
             transfer performance. But makes connection unsecure.
+        prefixes (tuple): Tuple of prefixes to force use.
     """
 
-    def __init__(self, storage_parameters=None, unsecure=False):
+    def __init__(self, storage_parameters=None, unsecure=False, prefixes=None):
         # Save storage parameters
         self._storage_parameters = storage_parameters or dict()
         self._unsecure = unsecure
@@ -31,7 +32,10 @@ def __init__(self, storage_parameters=None, unsecure=False):
         self._client = None
 
         # Initialize prefixes
-        self._prefixes = self._get_prefixes()
+        if prefixes:
+            self._prefixes = prefixes
+        else:
+            self._prefixes = self._get_prefixes()
 
     @property
     def client(self):
@@ -211,6 +215,16 @@ def prefixes(self):
         """
         return self._prefixes
 
+    @prefixes.setter
+    def prefixes(self, prefixes):
+        """
+        Set URL prefixes for this storage.
+
+        Args:
+            prefixes (tuple of str): URL prefixes
+        """
+        self._prefixes = prefixes
+
     def relpath(self, path):
         """
         Get path relative to storage.
diff --git a/pycosio/_core/storage_manager.py b/pycosio/_core/storage_manager.py
index 455e941..c4d6e27 100644
--- a/pycosio/_core/storage_manager.py
+++ b/pycosio/_core/storage_manager.py
@@ -72,7 +72,8 @@ def get_instance(name, cls='system', storage=None, storage_parameters=None,
         if same_parameters:
             return info['system_cached']
         else:
-            return info['system'](**system_parameters)
+            return info['system'](
+                prefixes=info['prefixes'], **system_parameters)
 
     # Returns other classes
     if same_parameters:
@@ -145,6 +146,8 @@ def mount(storage=None, name='', storage_parameters=None,
     if extra_url_prefix:
         prefixes = list(prefixes)
         prefixes.append(extra_url_prefix)
+        prefixes = tuple(prefixes)
+    storage_info['system_cached'].prefixes = storage_info['prefixes'] = prefixes
 
     # Mounts
     with _MOUNT_LOCK: