I just got an external drive attached to my Wii U. My original intention with the setup was to keep the games themselves on the hard drive, with their save data in the system memory (since hard drives aren't the most reliable things on the planet some times). However, the copy/move function only lets me move the entire thing (game+save data) as a set. And if I try some trickery to split them up (I copied the NSMBU data to both locations, then deleted the game data from the memory and the save data from the hard drive), the game acts like there's no save data at all. Seems a little odd to me.
And to top it off, after figuring this out, I moved the save data for NSMBU back to the external hard drive...and the game download plus its update data were deleted. What's up with that?