I have a modded Wii U that I set up a few years back so it's all a little fuzzy on steps and specifics.
I have an SD card and a USB hard drive.
I am trying to add a Wii U game (Resident Evil) and its 13gbs so it won't fit on a FAT32 SD card.
I have it in WUX format, WUD format , and in a folder...