you dont want to let the wii u format it btw
the wii u expects that connected hard drives will have to be in the wii u's proprietary encrypted filesystem format, it's not expecting fat32 or ntfs drives to be used (because those are only a piracy thing for wii side)
so when it sees a drive connected that isnt in the wii u format, it wants to format it before it's usable as usb storage
you can use ustealth to hide the drive which will prevent wii u menu from seeing it, while usb loader gx can still use it