1. A method of operating a data identification system for identifying data items in a storage system, the method comprising: receiving a scan request to scan files on a secondary storage volume; identifying changed segments of a primary storage volume, wherein the primary storage volume comprises a virtual disk file; wherein the identifying the changed segments of the primary storage volume comprising the steps of: generating a request for a changed segment list; transferring the request for the changed segment list; receiving the changed segment list; processing the changed segment list to determine the changed segments of the primary storage volume; identifying changed data items of the plurality of data items in the secondary storage volume based on a correspondence between the plurality of data items in the secondary storage volume and the changed segments of the primary storage volume; retrieving the changed data items from the secondary storage volume; and transferring the changed data items in response to the scan request.