You can, but you'd have to use a different drive. The Wii U's USB file system is encrypted and only works properly in Wii U mode.This is probably more my ignorance than anything else. It's a disk. Can I install a Wii disk onto a Wii u so I don't have to use the disk any more?
You technically can, using the buggy and unfinished FAT32 support in Mocha.I believe the question is can you mix both Wii U and vWii content on the same hard drive... and the answer is no, at least not yet. And don't hold your breath waiting for it.
Assuming you already have one external drive that the Wii U has already formatted for it's stuff, you'll need to buy a second external drive. Using UStealth you can "hide" the secondary hard drive from the Wii U, this will prevent you from accidentally formatting that drive as well. Dump all your Wii games on there.