In a RAID system, the power supplied to hard disks (HDDs) can be increased, and a plurality of types of HDDs can be installed. A plurality of HDD packs 33 are supplied with a single high voltage from a motherboard 28. The HDD packs 33 each accommodate, in a canister, an HDD 107, 181, 185 or 187 with different power supply specifications or communication interface specifications, as well as a DC/DC converter 109 to convert the power supply. Part of the HDD packs 33 have a data transfer interface conversion circuit 195 as well.