A disk array system including a plurality of disk drives, including a plurality of first-type disk drives having a first predetermined level of reliability and used to form a first-type logical unit having a plurality of a first-type of chunks, and a plurality of second-type disk drives having a second predetermined level of reliability which is lower than the first level of reliability and used to form a second-type logical unit having a plurality of a second-type of chunks; and a storage controller which copies data stored in a source chunk to a destination chunk, wherein the destination chunk is selected from the first-type of chunks or the second-type of chunks.