The problem is that your HD7850 doesn't have UEFI BIOS extensions and the system can't initialize/enumerate it to the operating system during the UEFI mode boot sequence.
AFAIK, the only add-in video card that has UEFI BIOS extensions available are certain models of the nVidia 600 series. I haven't seen any mention of ATI's support for UEFI. I ended up ditching my Radeon HD5850 for this reason. The Intel HD4000 integrated GPU has been more than adequate for me, but I'm not a gamer.
If you really need to use the ATI/AMD video card your only option at present is to install Windows in legacy BIOS mode.
Thanks, I finally got a sensible answer
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Maybe the message you saw is related to this:
Secure Boot requires a graphics card with a signed option ROM - we can't launch the opROM if the code is unsigned (security hole).
@rbmorse Legacy BIOS mode is not the only option, he can use UEFI Boot but he must have Legacy Boot enabled as well, for the graphics card to work. But the best is to continue using IGP to boot Windows 8 and use ATI for games.
I'm using now IGP and Lucid Virtu MVP. It works, but I do not like when things are not as it should work. I sent an inquiry to Asus about this problem. Within 48 hours, promised to answer.
When I turn on Legacy BIOS and UEFI together, the error is not detected.
ASUS said that the problem is in the motherboard. Their card supports Uefi.
I see that there is Radeon cards with UEFI GOP support.
MSI, Sapphire and VTX3D also offers hybrid vbios for their Radeon cards, on request.
But it seems that ASUS has not released UEFI VBIOS for their cards. Shame.