A storage device has plural data disks including a primary data area and a backup data area. Performance and reliability are secured while conserving power. A system management means includes a disk rotational state detection means, a disk rotational state control means for rotating or stopping a data disk, and a data placement control means for accessing the data disk to move the data. The data placement control means, if the data disk of the primary or backup side has been stopped at writing time, spins up and accesses thereof, and if the data disk of the primary or backup side has been stopped at reading time, prioritizes the side that is being rotated and accesses thereto, and if the data disk of the primary and backup side have both been stopped at reading time, spins up and accesses the side that has been stopped for the longer time.