Proposed is a disk controller capable of notifying that a hard disk drive needs to be replaced even when the power of such hard disk drive is turned off. When a controller is to control the supply of electrical power to a disk in the respective disk units, the controller turns off a green LED and turns on a red LED when a disk in a disk group becomes a replacement target disk, turns off the power of the replacement target disk by opening a power switch corresponding to the respective replacement target disks, turns on the disk power source by closing the power switch when the replacement target disk is replaced with a new disk, and recognizes information concerning the new disk.