Upon starting the initial copy process, a pair creation command is issued from a primary disk subsystem to a secondary disk subsystem. The secondary disk subsystem sets a target area of a second disk drive to an unwritten status, and thereafter writes update data that was updated in a primary site into a designated area of the second disk drive. Subsequently, when data of a first disk drive is backed up to an external medium, the external memory medium is transported to a secondary site, and then connected to a host computer. Unwritten data among the backup data recorded in the external memory medium is thereafter restored to the first disk drive, and the initial copy is completed on the condition that all data in the first disk drive has been copied to the second disk drive.