While I am a Windows 7 person, I think you can boot off of an external platter-driven hard-drive. If the ext HDD's boot partition(s) been marked Primary and Active, if the ext HDD's Windows partition is Primary -- that just might boot. However, I'm aware that "Windows 10's UEFI" is not at all the same as my "Windows 7's BIOS" -- so quite possibly, all bets are either off or going to be quite difficult.