What the above poster said but I'm assuming that you're done using it for Wii U right? Because you can't have it working on both at the same time it's one or the other.
Right click your start button then click Run at the bottom, type in diskmgmt.msc
Look for the drive in the list, it will have Disk # next to it like Disk 3, make note of this. Close the disk manager.
Right click your start button then click Run...