|Depending on your BIOS version you may not be able to boot USB. On mine it won't boot USB drive, nor do you get a choice to unless you first put the USB drive in the slot before powering up. Only then does it find it and allow that choice to select from.|
So I get a different boot choice menu depending on if I have a drive in the slot or not. Mines a Dell but I have no clue what make and model you are trying to use. So this is kinda of a waste until you post that info.