I cannot seem to access the bios at all with windows 8.1 and my gtx 780 ti. When I connect the hdmi to my gtx 670, I can see the bios screen and can change the config but with the hdmi in my 780 ti I get a black screen until the dual boot windows selection. I've disable fast boot and have no idea what else to do.
My guess is its defaulted back to another output on that card. I have three monitors on my GT 640. My BIOS splash screen and the Windows splash screen display on one monitor (DVI), and the Windows logon screen etc display on my main monitor (HDMI). It does the same thing whether I set Windows to single monitor or extend. I haven't tried it with just the HDMI monitor plugged in by itself though. Normally they should all go to the one monitor that's plugged in. If I get a chance latter on I'll unplug my two DVI monitors and see what happens. There is no guaranty that it will do what yours is doing but it will be interesting to see what happens just for future reference for me.
Ok, I just unplugged my two monitors that are connected via DVI and left only the one connected to my HDMI port. Everything was visible on that monitor including the BIOS splash screen.