Another thought, if you put that USB (or the different one you're working on creating now) in a different PC and try to boot from it, does that work?
You of course don't have to fully install it or anything, just starting up till the installer to make sure the USB is fine should be good enough (maybe up till the disks and network to see if it can find both if you want to be extra sure).
Helps us to figure out if the problem is with the (process of creating the) USB, or with the device itself.