Nintendo recommends using self-powered hard disk drives, i.e. devices that use an external power supply instead of drawing power through USB. On the other hand, some bus-powered hard disk drives draw more current than the USB 2.0 specification and might not run stably. So we cannot guarantee that such devices will run properly on the Wii U without the use of a supplemental power cable called a y-cable that draws power from two USB ports. For the best results with storage devices, we recommend using a hard drive with its own power supply that connects to the Wii U via the USB port on the console.