I've been using an external drive to save my games ever since I bought my Wii U. Everything was working very well until the week before Sm4sh came out. I turned on the console and despite the drive spinning and the light coming on, my games wouldn't appear on the screen, and the console didn't even seem to think there was a Wii U formatted drive plugged in.
I tried turning the console on and off a few times, switched USB ports, but nothing worked. The internet seemed to think that the problem came from a game update during one of those standby functions. In fact, When I checked the download menu, there was a failed update for MK8 which probably corrupted the drive. I tried deleting it, but it still didn't help. I agreed that the only thing to do was to have the Wii U reformat the drive. I was kind of sad for all my game progress, but I figured that these things happen and it would have been much worse if it were a different console or my pc games. I also will never use standby functions again. I really loved having ready to play, updated games when I wanted them, just like Steam.
Everything works now, but I've got an annoyance. Sometimes when I start up the console, all the games will be there, very nice. Other times the screen grays and the hard drive symbol comes up in the corner. Then it goes away, but the games still aren't there. I know from experience that I should just leave it alone until it gives a don't plug or unplug drives in while the console is powered error (even though I haven't touched the drive) and then soft resets. It then works. So it always works, I just get annoyed because sometimes I have to wait for a minute or two because it seems to think the drive was unplugged.