I don't know. Some mobos will boot off USB, some will not, I don't know yours. You could
1) shut down and power off
2) Attach said external hard drive (does it have a bootable partition?)
3) Interrupt the boot sequence and enter the BIOS (is this familiar to you?)
4) Go to the list of drives and see if the external one is recognized
5) Go to the boot menu and see if the external one can be set to the boot device.
and let us know what you see.