Numeral 11 denotes the control unit, 11a the MPU (a microcomputer), 11b the RAM cache, 11c a disk interface (SCSI protocol controller SPC), 11q a register in which power-supply cut-off information is set, 13 the operator's panel having a display unit and various operating keys, 13c write-in notifying means such as an LED by which the operator is made to recognize that data is currently being writt