With the drive connected, press F12 a few times at powerup.  A menu will appear, among which will be the USB drive (as long as the system sees it as bootable).  
I need to boot from a USB, but no USB drive appears in the boot menu. If I insert a bootable USB in a slot and then boot the system, will it automatically boot from the USB? If not, how do I have the system check for a USB?
How do I boot from a USB?