Asus P8Z77-M boot device choice
I found that there was no known special hotkey that will allow one to choose which device to boot when powering-up the PC.
What I found was to keep pressing Delete key upon booting, which brings you into BIOS setup. Under the Boot tab, scroll to the bottom, where you will see “force boot” choices. Click the one you want and the PC will do a one-time boot from that device.