Boot to the system BIOS and go to the Boot section. Use the right side navigation to go down to +HDD and expand that. You will then see the list of devices as shown in the pics below. Select the USB drive and move it to the top of the HDD list. Reboot and thats it. This will not stay once you remove the USB drive, it will resort back to the default of booting from the hard drive.

LS800BIOS000