After a lot of trial and error, I finally got my Wii U to play backups off my external HD, using Wiiflow. Since you can only have one HDD plugged in at a time, I switched it out for my usual Wii U HDD, which I use for Wii U games. I felt like playing a Wii game the other day, so I hooked it up, and tried to play some games with Wiiflow. All the games show up in the selection screen, but every game just boots to a black screen, and stays stuck there. This is even on games that worked perfectly fine a week earlier. Edit: I used USB Loader GX r1207, everything works great now.