To anyone stumbling across this thread, or to OP if they are still having difficulties. I just solved the issue here myself.
When you first boot into the EFI Shell, run this command:
map -b
This will map the drives connected to the computer, from there you'll want to select your USB device. Which will most likely be fs# (where # is a number).
You can access that drive by typing
fs#:
From there continue with the rest of the steps, and you'll be golden.