In order to boot from the USB / DVD we need to disable the 'Fast BIOS' mode and change the Boot Device Priority:
1. Shut down the laptop.
2. Press the power button, then keep pressing the F2 key until you are taken to the bios menu.
3. Go to Advanced tab and disable the option 'Fast BIOS mode'.
4. Go to Boot tab and change "Boot Device Priority" so USB HDD is at the top.
5. Save the changes and exit. then shut down the laptop.
6. connect the USB, turn on the laptop and press the F10 key straight away, this will bring you to the boot menu to select the USB.
See how to do it in this video: