The invention is a storage device (1) for a host computer system. The device (1) incorporates a Supervisor function for controlling access to information stored in a storage medium (2) of the device. The main embodiment described is a hard disk drive (1) comprising: one or more disk platters (2) for storing information; a ROM (4) for storing firmware for controlling operation of the drive; a volatile RAM (5); a micro-controller (7) for controlling the transfer of information to and from the disk platter(s) (2); and an interface (6) for interfacing the drive (1) with the host computer system and via which information is transferred to and from the disk platter(s) (2) under the control of the micro-controller (7). A Supervisor is provided in the form of firmware which is preferably stored in the ROM (4), the Supervisor operating the micro-controller (7) so as to protect information stored on the disk platter(s).