Commands are provided for storing data in a plurality of different types of storage media, thereby eliminating the storage errors of important data and improving usability. A storage device comprises a host interface, a hard disk interface acting as an interface to a hard disk, a flash memory interface acting as an interface to a flash memory, a command analyzing section which analyzes the contents of a command inputted from the host interface, and a data write operation processing section which performs a data write operation on both of the hard disk and the flash memory when the command analyzed in the command analyzing section is a predetermined command for write-through acting as a command for writing data into a plurality of media.