A mobile hard disk drive includes an electrical module including a circuit board, a face panel and a holder box, and a hard disk tray detachably insertable through an insertion slot in the face panel into a receiving chamber in the holder box to force arched spring plates thereof into friction engagement with two opposite inner lateral walls in the receiving chamber of the holder box and lockable to the face panel by two hand screws that are mounted at an end plate of the hard disk tray for threading into respective locating screw holes at the face panel to lock the hard disk tray to the face panel of the electrical module.