My guess is the BIOS on those laptops does not support drives over 1TB capacity.
There may be a BIOS patch or update from the laptops' respective manufacturers which removes that limit, but don't install any such updates unless and until you know they do actually address that particular problem (read the accompanying notes on their BIOS download page).
Because updating the BIOS can be potentially disastrous if something goes wrong, you need to have a good reason for doing it).
I did go and get the latest bios update for this laptop. I am now able to boot up to the USB drive. I found that like on HP laptops, u can set a delay so that it can read all drives before booting up, this model of dell did not have that option, still doesnt after the bios upgrade, so it must have been programmed in as i can now see my usb drive as a choice for bootup