A large capacity HDD is handled as a portable recording medium. In a state where a portable hard disk (PHD) unit is mounted on a cradle, data is written and read between the PHD unit and a host device. By having a first engagement section on the side of the PHD unit and a second engagement section on the side of a mounting section of the cradle mutually engaged while the PHD unit is mounted on the mounting section, a state where the PHD unit is mounted on the mounting section is maintained.