I'll keep this short and sweet. I was having trouble booting from a USB stick, and was convinced that my laptop just didn't have the capability. I am happy to have learned recently that I was wrong, oh so wrong!
How to boot from a USB stick (that has been formatted with a boot loader or a rescue OS):
1. Shut down the laptop. Cold; all the way
off, because "restart" won't do it!
2. Plug in the boot stick.
3. Boot, and press F2 when the ASUS logo appears.
4. Enter BIOS password, then go to "add a boot device".
5. The USB stick should now show up; add it, move it into correct order, save and reboot.
Your G73 should now boot from the USB stick.
I have not tested it but the above process should also apply to USB 2.0 and USB 3.0 hard drives that may be connected either directly or via a hub.
My Current Addiction: Asus G73SW i7
12 GB RAM
Windows 10 Pro
3TB internal, 5 TB external via USB 3.0
^ I use this system as a mobile DAW, 3D graphic studio, and for other stuff.