An electrical host system includes a host and an expandable optical disk recording and playing device. The expandable optical disk recording and playing device includes an expanding interface module, an expanding interface, a storage interface module, an output interface module and a CODEC module. The expanding interface module connects to the expanding interface and the host. The storage interface module connects to a storage device. The CODEC module encodes, decodes or transcodes an audio/video source to generate audio/video data, wherein the audio/video source is inputted from the host through the expanding interface and the expanding interface module. The audio/video data are outputted through the output interface module, or through the storage interface module to the storage device.