UPDATE (in case anyone has this problem in the future): I was about to give up on teconmoon injects.
I unplugged my Y cable from my Wii U, and reversed the two USB ports that I was plugged into on the Wii U (i.e. I previously had USB A plugged into Port A and USB B plugged into Port B, but i reversed this so USB A was plugged into Port B, and USB B was plugged into Port A).
Works perfectly now!!! No clue why it didn't work before (but everything else worked fine moving it to the USB drive), but whatever!
I unplugged my Y cable from my Wii U, and reversed the two USB ports that I was plugged into on the Wii U (i.e. I previously had USB A plugged into Port A and USB B plugged into Port B, but i reversed this so USB A was plugged into Port B, and USB B was plugged into Port A).
Works perfectly now!!! No clue why it didn't work before (but everything else worked fine moving it to the USB drive), but whatever!