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I took a broken card from 2011. I replaced the GPU and want to test. Don't have Imac 2011 27 and I tested to 2010 27 but it takes 30 seconds
. First black screen and then after 30 seg it appear the desktop. Later work ok. Is that normal?
EFI part of that BIOS is not working and it will only work once OSX is booted and is using the PC part of the VBIOS. I made that 8006 bios and it should work if you got a real chip. Very possible you got a fake remarked chip.