A storage unit connected to other storage units, each having of first disk drives, includes second disk drives; a first receiving unit receiving copies of first storage data, stored in storage blocks created by logically partitioning a data storage area of the first disk drives, and first identifiers identifying the storage blocks, from the other storage units; a first operation controller calculating an exclusive OR of the copies of the first storage data, with a correspondence established among the first identifiers, from the copies of the first storage data received by the first receiving unit; and a first storage controller storing a calculation result of the exclusive OR from the first operation controller, into storage blocks of the second disk drives having second identifiers that correspond to the first identifiers and that individually identify storage blocks created by logically partitioning a data storage area of the second disk drives.