power supply unit is stopped a cache memory having first and second memory units and recording a part of data stored in a storage medium and a controller controlling power from the auxiliary power supply unit to the device and stopping power to the first or second memory unit one after another application_number US 3762108 A description an embodiment will be described below with reference to the drawings fig2 is a diagram for explaining the embodiment and a configuration diagram of a disk array device which incorporates a cache memory a disk array device 1 is connected to a host apparatus such as a personal computer via a network line in the same drawing the disk array device 1 is composed of power source units 2 and 3 monitoring units 4 and 5 controllers 6 and 7 and battery units 8 and 9 according to an aspect of an embodiment the disk array device 1 has two controllers 6 and 7 each of which is connected to a hard disk hd to be described later for example the disk array device 1 may be provide with not less than three controllers which transmit and receive data to from more hard disks hd a power is supplied from an external power source to the power source unit 2 or 3 and the power source unit 2 or 3 supplies the power to the components of the device on the basis of the power supply furthermore the monitoring unit 4 or 5 monitors the power source unit 2 or 3 and.