A storage system is described and includes a storage apparatus for storing data used by an external apparatus, first and second temporary data storage units, a host interface, a disk interface, and first and second controllers. The first controller is configured to select as a data transfer process, when the host interface receives a command from the external apparatus, one of a first data transfer process and a second data transfer process based on the command. The first data transfer process is a data transfer from the first temporary data storage unit to the external apparatus by the host interface. The second data transfer process is a data transfer from the first temporary data storage unit to the second temporary data storage unit by the second controller, and a data transfer from the second temporary data storage unit to the external apparatus by the host interface.