According to PsyBlade and KDH in this thread, it's possible.
Here's the approximate way of achieving this:
Many drives can be set up to appear shorter (Host Protected Area -- HPA). So, the main idea is the following:
- Set HPA on your HDD.
- Format the HDD on WiiU.
- From your PC, remove HPA, then write a partition table at the end of the drive, in the space that was protected by HPA.
- Set your linux kernel to read the partition tables at the end of the drive.
- Set HPA each time you're using your drive on WiiU, and remove it each time you're using it on Linux.
The precise explanations are detailed in this post (but the whole thread is informative).
I take no responsability whatsoever for any loss of data consecutive to you trying to follow my explanations. Play safe, backup your data before attempting any partitioning task!
I would normally refer to the printed manual, which we do of course have access to in English, but it refers to the electronic manual for more information about USB devices, and this manual only seems to exist on the Wii U itself.
I don't have a Wii U yet (partly because it isn't out here yet), but I would expect that this information is also repeated there, in which case an American Wii U owner should be able to verify this.
At any rate, I watched the video, and based on my limited knowledge of Japanese, this seems to relate to the notice shown at 8:16:
大きな電流が流れてしまう可能性があり
「Yケーブルタイプ」でないと
Wii Uでの安定動作が保証できない
If my translation is reasonably accurate, they're essentially saying that drives requiring a Y-cable connection (an extra USB plug for power) may not function correctly. 2.5" drives which don't require this should be fine (note that some drives essentially negotiate for more power over a single port; I suspect that the Wii U does not support this, so those drives would neeed a Y-cable to get 1A).
Most likely, they're just being cautious, and your drive will work fine, but it is possible that there is some edge case where the Wii U is unable to power both ports and ends up disabling the "extra" port.
However, do note that this is entirely based on the text in the video - I don't understand spoken Japanese, so Iwata may be saying something additional here which affects the meaning; and there is the possiblity that I mistranslated something.
Best Answer
According to Nintendo support, a dedicated power source is not necessary, but recommended.
Emphasis mine.
From the above quote, it appears that although the use of a y-cable is already good enough, you might still encounter a drive requiring more power than even two USB 2.0 ports together can officially provide.