If a drive only has one plug and doesn't work, then it's because it's not following the USB specifications. That's the fault of the drive for expecting out-of-spec voltage if the Wii U's supplying power according to the 2.0 spec (500ma, 5V).Update: Bus-powered USB hard drives will work, but some will need the Y-cable USB connector. That's a relief, and I doubt the MyPassport HDDs draw that much power.
http://www.ign.com/articles/2012/11/18/nintendos-steams-up-the-wii-u-eshop
The whole "two plug" thing is to get around that issue, to let devices pull power from two plugs at once so they can get what they need.