Read errors in a plurality of hard disks are detected at an early stage, and a redundancy is maintained when one of the hard disks breaks down. A hard-disk selector selects by priority a hard disk having a read error during a patrol. A verifying unit reads a predetermined amount of data from the hard disk selected. An error detector determines whether the read error has occurred. When it is determined that the read error has occurred, a replacing unit secures a spare area, and stores the corresponding data in the spare area.