A disk array system including a plurality of disk drives, including: a plurality of first-type disk drives being used to form a first-type logical unit having a plurality of a first-type of chunks; a plurality of second-type disk drives being used to form a second-type logical unit having a plurality of a second-type of chunks; and a storage controller, if the storage controller copies data stored in a source chunk to a destination chunk, selecting the destination chunk from the first-type of chunks or the second-type of chunks.