i use one of these a WD 320GB USB one http://goo.gl/FM6IT it is very small & fits nice on top of the wii u you will also need a Y cable to connect it to the wii u to get enough power http://goo.gl/OFHov it is the same with many external HD's for the wii u so don't worry about using a Y cable.
i was shocked when i read your post. i have the same external HD & it doesn't work with regular cable or with the y cable. please any advice it will be welcomed