DATA MULTIPLEX METHOD

PROBLEM TO BE SOLVED: To improve the fault resistance of restoration copying for restoring multiplex in a data multiplex system and to maintain the matching of copy data. SOLUTION: In restoration copying for maintaining data multiplex between a master system 100 and a remote system 110, former data on a disk drive unit 113, which corresponds to differential data, is saved to a cache memory 114 or a parity track 220 or a spare disk 230 before overwriting and differential data are stored in the target storage area of the disk drive unit 113 on the side of the remote system 110 receiving differential data from the master system 100. When restoration copying is failed owing to the fault of a master system 100-side, former saved data are returned to the disk drive unit 113 and the matching of copy data is maintained.