Embodiments in accordance with the present invention provide a disk unit capable of restricting access to data stored in the recording medium depending on users. In one embodiment, the user enters a password into a host computer. This password is transferred to a password verification section. Then, the password verification section checks whether the password coincides with any one of authentication passwords stored in an authentication password storage section. If the password coincides with any one of the passwords, the password verification section supplies an authentication signal to an operation mode control section. The operation mode control section supplies an operation mode signal to a controller by receiving the authentication signal. The controller controls execution of the operation mode according to the operation mode signal.