A method for operating a computer data storage system stores snapshots of an active file system of the storage system at a plurality of destinations. A latest snapshot stored at each destination of the plurality of destinations is identified. Those data blocks which are newer than the latest snapshot stored at the each destination are sent to the each destination. The active file system is scanned to find each data block newer than the oldest snapshot stored at a selected destination, and all such data blocks are tagged. Those data blocks which are tagged are sent to the selected destination.