During my research for this question I learned that booting from a USB flash drive (UFD) can be a problem. Apparently, there is no standard accepted way for a BIOS to recognize and boot from a bootable UFD.
One solution is to recognize a bootable UFD as a hard drive, giving you the opportunity to temporarily switch the order of bootable hard drives in the BIOS. Other computers (e.g., Dell) provide the ability to press F12 at bootup to choose the bootable device
Does this make sense to you?