A storage device, a data arrangement method and a computer program are disclosed, in which the redundancy is maintained even if a cache module runs out of order, and the deterioration of the write processing performance is prevented at the time of failure of the cache memory module. When cache 0 fails while cache 0 and cache 1 possess duplicate data that is not stored in disks, data of cache 0 is copied onto data of cache 4 that is synchronized with the disks.