Just to add it might be a smart idea to back up your Wii u hard drive every so often in case the Wii U doesn't see anymore. If you need to reformat the drive you lose all your save data as the hard drive won't be read on a pc. Had that happen to me several times and learned the hard way.
Also Saves are part of the digital game data if you buy your retail games digitally. I have not found a way to save the save data separately.