1. A storage system that receives access commands from an upper level device, comprising: a plurality of storage devices; a volatile cache memory; a non-volatile memory that is a type of memory that can continue to memorize data irrespective of whether or not power is supplied; an access control unit that, when the access command is a write command, temporarily memorizes the data following the write command in the volatile cache memory, and reads from the volatile cache memory and stores the memorized data in at least one of the plurality of storage devices, and that, when the access command is a read command, reads the data following the read command from at least one of the plurality of storage devices, temporarily memorizes the data in the volatile cache memory, and then reads from the volatile cache memory and transmits the memorized data to the upper level device; a battery that is a secondary power source; and a cache data copy unit that, when power is no longer supplied to the volatile cache memory from the primary power source, copies the data memorized in the volatile cache memory to the non-volatile memory based on power supplied from the battery, wherein when power is supplied from the primary power source to the volatile cache memory, the cache data copy unit specifies, from among a plurality of volatile sub-areas included in the volatile cache memory, LRU volatile sub-areas, and copies the data in the specified volatile sub-areas to the non-volatile memory.