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 .