But the 360 already had sata ports which makes it extremely simple for someone to install a hdd internally instead of the dvd drive. PS3, all you need is a male to female sata cable and you're good to go as well and providing you have an external power source, there is no soldering needed. However this guy does not sound like any kind of an expert (no offense OP, not meant to insult you) and I was not picking at him in the least as I thought of this as well, however after further investigation, there is no sata ports, therefore no way to connect a drive natively.
This also means that most likely, the OS on the Wii U will not recognize an hdd connected internally even if you managed to frankenstein one in without some sort of alternative code for it. For that you would need a CFW/some other kind of softmod, which is not available yet.
So, OP, I was not being an ar$ehole, I was truly interested as to how you would accomplish something like this and to what end/goal you hope to achieve. Care to enlighten us?
EDIT, just re-read OP. Pretty sure it didn't say anything about a USB prior--sneeky OP
At any rate, if you're going to have cables running back outside to connect to the usb ports, what's the point? It will look absolutely fugly, not to mention voiding your warranty for no advantage. not exactly the brightest thing in the world to do.
You would be better off building a custom case for your external drive that matches the Wii U and using white cables vs black.....