Method and system of storing a main data file and deltas in storage device fordetermining new data files from the main data file and the deltas

A storage system and method for storing delta difference data indicative of the differences between new data and old data. A data transferring unit has copies of old data and new data. The data transferring unit compares the new data with the old data to determine delta difference data indicative of the differences between the new data and the old data. A target storage device having a copy of the old data receives the delta difference data from the data transferring unit. The target storage device stores the delta difference data and may then determine the new data by merging the old data with the delta difference data. The target storage device transmits the determined new data to a data receiving unit upon request. The method and system may be used with peer-to-peer remote copy (PPRC) storage system and storage area network (SAN) storage system.