Doesn't work with usb external hard drive?

[admicos]

Well, i'm not EXACTLY sure if it will work or not, since i'm only testing it on my computer (need it on other computers later so i tried to make sure it works)

So, i can get on the main screen, but trying to boot USB only returns with a error. (Also tried holding SHIFT, and got a different error).
I also tried plopkexec but it only detects itself.

(I am sure the drive i'm trying to boot actually does boot.)
(everything was tried on the BIOS compability mode in a UEFI board, so that might be a part of the problem too.)

Hopefully it actually works on the other computers tomorrow, since they are pretty old.