Provided is a disk array system which is connected to a computer and which data is transmitted by the computer, including: a plurality of disk drives for storing user data transmitted by the computer; a cache memory for temporarily storing data sent/received among the computer and the plurality of disk drives; and a control unit for controlling input/output of the data to/from the cache memory, in which the control unit stores user data identification information that is capable of judging whether the user data is stored in a sector of the disk drive. The disk array system according to this invention allows the identification of user data of a disk drive.