While performing data transfer for reading data from a source disk or disks and writing the read data to disks of a disk array, data reading is repeated until accumulated read data can be split into several sub-data with a data size of each sub-data not smaller than a write maximum. Then, these sub-data are written into the disks of the disk array. In this way, the invention can take full advantage of each writing control to write as much data as possible, and the number of writing controls can be reduced to reduce the demands on the computer system.