An expansion device includes a peripheral component interconnect (PCI) card, a hard disk connector, a hard disk drive (HDD), and a signal converter. The PCI card includes PCI pins for connecting to a PCI slot of a motherboard of a computer. The connector is arranged on the PCI card. The HDD is attached to the PCI card and connected to the connector. The signal converter is connected between the hard disk connector and the PCI pins. The signal converter receives a PCI signal from the PCI slot through the PCI pins, and converts the PCI signal to a signal suitable for and provided to a HDD through the connector. When the HDD completes reading and writing, the signal converter receives a feedback signal from the HDD through the connector, and converts the feedback signal to a PCI signal for being provided to the motherboard through the PCI pins.