A disk array includes a buffer to temporarily store data (data X) from a host, a redundant first and second disk having a mirror configuration, and an auxiliary disk. A controller controls reading and writing of data X in response to a request. The controller is configured to: (a) copy to the auxiliary disk, data X stored in the buffer for rebuilding, in parallel with reading and writing data X, when the second disk fails; (b) log copied disk locations of data X copied in (a); (c) return a response to the host after completion; (d) reference a redundancy management table before copying in (a) to determine whether data X stored in the buffer has been copied; and (e) bypass copying in (a) and return to the host a response when current data X has been copied in (d).