diff --git a/interfaces.py b/interfaces.py
index a49ce22..52a092f 100644
--- a/interfaces.py
+++ b/interfaces.py
@@ -786,9 +786,12 @@ class IExternalFile(IFile):
                missing_value='',
                required=False)
 
-    def copyDataFile(self, targetAddress):
-        """ Copy the external file object with all data to the target address
-            given.
+    def copyDataFile(self, targetParams, targetAddress=None):
+        """ Copy the external file object with all data to the destination
+            specified by target params and target address.
+
+            If no target address is given the external address of the
+            object is used.
         """
 
 
diff --git a/resource.py b/resource.py
index b221e33..e317767 100644
--- a/resource.py
+++ b/resource.py
@@ -402,6 +402,10 @@ class ExternalFileAdapter(FileAdapter):
     def storage(self):
         return component.getUtility(IExternalStorage, name=self.storageName)
 
+    @Lazy
+    def modified(self):
+        return self.storage.getMTime(self.externalAddress, self.storageParams)
+
     def copyDataFile(self, targetParams, targetAddress=None):
         storageParams = self.storageParams
         externalAddress = self.externalAddress