Currently I'm at a LAN party and this morning I connected my Razer Tartarus to my PC. When I rebooted, suddenly I was unable to select any GRUB entry (my keyboard didn't work).
It worked fine yesterday and this morning, so what happened? My gut told me to try to use my Tartarus (which basically is a keyboard) and voila, that worked. Then I connected the Tartarus to my non-onboard USB ports and my Das Keyboard Model S Ultimate to an onboard USB port and voila, the keyboard priority changed back to my normal keyboard again.
Strange that the BIOS (UEFI) only can handle 1 keyboard at a time... Maybe it is a limitation of my Asrock Z97M OC Formula, who cares. Just glad I debugged this so quickly.