disk units operable under control of different disk control units hold the same data . under circumstances in which data is duplexed , when data is duplexed again after data that is generally saved as backup data was accessed for reading and writing , in order to efficiently duplex data , update places of both original data and sub - data updated during the duplexing of data is interrupted are registered on an access information management table of an original disk control unit . when the original disk control unit receives a duplexing resume command , the duplexing of data may be efficiently reorganized by copying original data corresponding to an access place produced during the duplexing is interrupted from information of the access information management table to the backup data or by copying the backup data to the original data .