On the Nintendo official website, they suggest an external hard drive with a separate power source to be used with Wii U. Does that mean a pure USB hard drive wont work?

Can bus-powered hard disc drives be used with Wii U?
Some bus-powered hard disc 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 Wii U without the use of a supplemental power cable called a y-cable that draws power from two USB ports. For best results, we recommend using a hard drive with its own power supply that connects to the Wii U system via the USB port on the console.