I am getting the same thing for the past two days. I dont think its your fault. I don't leave mine on as long as you and I still have the issue.
I finish Mario Kart 8, Close and go back to the Wii U Menu, but it gets stuck on the white screen with the Wii U logo and Wii U Menu text right below it, both on my TV and gamepad. I do have a Western Digital External HD (250 GB) connected. It happened last night and just now again.
What's going on, Nintendo?