1. A method of operating a data identification system for identifying data items in a storage system, the method comprising: identifying changed segments of a primary storage volume; receiving a data request for a plurality of data items in a secondary 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; and transferring the changed data items in response to the data request.