Cleared All Contents of Wii U now USB not detected

Hello,

So I was having problems with Mario Kart 8 constantly freezing during gameplay so I decided to copy all of my game data onto a USB stick and cleared all contents of my Wii U in hopes that it will fix the problem.

However, when I stick my USB stick into the Wii U, it does not seem to detect it anymore. Do you know what is going on? Is there a way to recover my game data from the USB stick and load it back onto my Wii U?

Thanks