A computer system has an internal mounting unit in which a device supporting a predetermined interface is mounted. The computer system includes an external mounting unit supporting the interface; a device controller comprising a plurality of communication channels, which can communicate with the device, for communicating with the device mounted in the internal mounting unit through a first communication channel of the plurality of communication channels and communicating with an external apparatus mounted in the external mounting unit through a second communication channel of the plurality of communication channels; and a switching controller for selectively connecting the external apparatus mounted in the external mounting unit to one of the second communication channel of the device controller and the device mounted in the internal mounting unit according to power-on/off of the system.