Can't say for sure, but I'm betting you have a hard drive that the Wii U needs you to either plug into an outlet for power, or for it to have one of those cords that connect it to 2 USB ports to get full power, either method should work. This is the case for many models of HDD. Mine works fine when plugged into a regular wii with just one cord, but in a Wii U, while it will power on, it for some reason doesn't receive enough power to save, read, and transfer to unless I use one of those cords that power up the HDD by connected to 2 USB ports.