I've been having a ton of trouble with my Wii U external HDD lately. It randomly will disconnect and after playing with it and troubleshooting it for months I've come to determine that it's probably the fault of the Wii U and not the HDD.
So I looked for a good solution, but didn't want to buy another external HDD with external power to just run into the same situation again
Then I found some other people in forums having a similar issue and I was super surprised to see the recommended fix.
Who would have thought the fix would be a cheap little flash drive? I grabbed a 128GB flash drive as cheap as I could find it (which was very cheap) and moved all the games from the old HDD to the flash drive.
Not only is it totally stable, it's also faster to load. Weird.
Anyway, if you run into the same problem maybe the same fix will help you.