So I kind of jumped the gun on getting a hard drive for the Wii U. I purchased a drive before Nintendo released their complete information about which drives are supported, and mine kind of falls outside their supported list.
Nintendo prefers that external drives be USB 2.0, and that they be externally powered via AC adapter. Provisionally, bus powered drives that use a two USB port Y-cable will also work.
In my case, I have a portable 2.5" drive that is USB 3.0 and uses a single cable. It's a 1TB Seagate Portable Expansion drive. Fine, I thought, I'll just buy a separate Y-cable to power it. Unfortunately, the port on the drive is not a standard Micro USB, but uses the USB 3.0 Micro B connector. Scratch the Y-cable. (Update: I have since discovered that there do exist Y-cables with the USB 3.0 Micro B connector, and I will be going out and getting one ASAP. Meanwhile, the rest of my test was performed with just the regular single cable.)
Alright, well, I'll just try it to see if it works, right?
Plugging in a drive while the Wii U is on brings up a warning to power down the console before pluggingin USB hard drives. Regardless, it still brings up the "Do you want to format?" window. Just to be safe, I power down and restart the Wii U.
Formatting the drive takes 3 seconds. And voila, it's ready for use. I try transferring a game from internal memory to the hard drive. In my case, it's Nano Assault Neo, the only download game I have. It's only 50MB, and it transfers over pretty quickly. After the transfer it appears on my main screen the same as always, and launches and plays without problems. I do notice that the load time is a little bit slower than before, but gameplay is smooth and there are no problems.
Emboldened by my experience, I decide to download another eShop game. You know, for 'testing purposes'. Sonic All Stars Racing Transformed is what I end up with. There's no other racers on Wii U yet, and it's $40. When I purchase, it doesn't ask me where I want to download it to, but when I check the Download Manager, it appears that it's downloading the 5GB file to my USB device.
That's interesting that it doesn't let me choose, because I have ample space on my internal memory. I would have thought it would start with internal first, and then switch to external storage when capacity was limited. For whatever reason, it doesn't. I can always move it later, I guess.
The download is going to take about an hour, it says, so I wait. So far, so good, things seem to work, at least with a small game like Nano Assault Neo. I'll come back and let you know how the larger game fares.
As always, your mileage may vary. Please note that Nintendo does not officially support external bus powered drives without a Y-cable.
UPDATE: The drive is not happy in its current configuration. The Sonic download stalled at about halfway, so I cancelled it. Just to check, I tried to transfer back Nano Assault Neo to the system memory. It can't do it, it gives me a "Cannot transfer" message. I can hear the drive spinning up and spinning down, repeatedly. So it looks like you really do need to heed Nintendo's advisory on hard drives! On the plus side, I am able to easily re-download purchases to my Wii U internal memory, so all is not lost. Well, except for my save data for Nano Assault Neo. Not a big deal, at this point.