As some of you may already know, certain USB self-powered external hard drive might not run stably with the Wii U due to lack of power in one USB port.
If you have a USB self-powered external hard drive that’s not working properly with your Wii U, you’re going to need a USB power adapter y-cable that draws power from two USB ports. SEE VIDEO!!
Apricorn AUSB-Y USB Power Adapter Y Cable – Black/Grey:
Coolmax 2.5-Inch Aluminum USB3.0 Hard Drive Enclosure: