A portable external hard disk system is provided. The hard disk system includes a hard disk unit and a conversion unit that is attachable to and detachable from the hard disk unit. The hard disk unit has an IDE interface terminal. The conversion unit has an IDE interface terminal and a USB terminal. Also, the conversion unit includes therein a secondary battery. The conversion unit supplies the combined power of the power of a USB power line and the power of the secondary battery to the hard disk unit.