Ah hmm, F8 was always what worked on my asus boards. Try F11 like the other chap suggests. I will reboot for you to see if I can figure out which button it is on my gigabyte board, and find the location in the bios for the boot selection (I am pretty sure there is a section called BOOT).
Ok I tried several of the F keys without any luck. However on my gigabyte board, if your using the new type bios screen (UEFI), there was a "BIOS" option on the main page of my board, I was able to change the boot order there, or if you use the classic bios (I press F2 to get this), I have a "Bios Features" menu, and at the top of the menu is the boot order.