The present invention creates real data and parity data using plural hard disk control units without changing the number of disk adapters per hard disk control unit and distributes and stores data in a hard disk. While it is being requested that a disk utilization rate is increased maintaining the failure resistance of the hard disk, and, at the same time, to support the combination of the real data+parity data as before, it is desirable that the number of adapters that comprise the hard disk control unit should not be changed. According to the present invention, the number of hard disks that comprise RAID can be changed without changing the number of disk adapters per hard disk control unit.