PURPOSE: To improve an access time which is viewed externally, to prevent write data from being lost owing to abnormality of hardware, and to improve its system efficiency by providing a nonvolatile memory for storing the write data temporarily.

CONSTITUTION: When data is written in a disk unit 4, write data transferred from a host CPU through a host path 7 is stored in the nonvolatile memory 6 under the control of a local CPU2 and a disk controller 3. The CPU2 accesses the address of a cylinder head and an address where the write address is written in its internal memory. Then, the distance between both addresses is compared with a specific value and whether the unit is in a writable state or not is checked when the distance is larger than the specific value; when so, the data stored in the memory 6 is written in the target cylinder. Consequently, the access time viewed from the host CPU is shortened, the write data is prevented from being lost, and the system efficiency is improved.