You may need to change the boot order to get it to allow you to boot up through USB.
NOTE: Before you set boot priority for a USB device, plug the device into a USB port.
To specify the boot sequence (as in what order the drives load in)...
1. Turn on the computer and then, during the initial startup (first or second screen), press ESC, F1,
F2, F8 or F10 (depending on which your device uses to get to BIOS).
2. Now enter BIOS setup. (It can mistakenly be called Aptio Setup sometimes because that is the title
in the screen, but this is BIOS).
3. Use the arrow keys and select the BOOT tab. You should now see "System devices" shown in order of
priority (which starts up when).
4a. To give a cd or dvd priority in the sequence...