Indeed, I purchased the card with the modified vBIOS (015.049.000.003.000000). Since I wasn't able to satisfactorily revert the OCLP patches, I decided to perform a "clean" installation of Big Sur. I created a USB with OCLP and a Big Sur installation, removed the old one
The installer allowed me, unusually slowly, to erase the SSD where I had Big Sur installed, but when it came to the installation, it seems like it doesn't start. I've tried it with two different USB drives and got the same response. I think I'm going to try installing Monterey to see what happens.
If I need to update the vBios, can it be done from a Mac or do I have to do it from a Windows system?
Thank you both very much for your help!
I got several failed attempts with total failure and black screen, had to power off the machine a dozen times, until I got back here and received direction to get the proper vBIOS. Your preflashed vBIOS might not be a good one, unless it acts like a native Apple card, with boot screen etc.
After getting the proper vBIOS, the installation went smoothly thanks to the boot screen, but it still took me a whole night for the iMac to set-up (it seemed to stuck at the last minute forever, so I went to sleep, leaving the iMac to work out itself)
You need both your iMac and another computer (I used my Windows PC) to flash.
Check the first post to get the link about how to flash, or search the post about GRML USB (a small Linux USB to plug in your iMac and boot from there)
Boot the iMac from the GRML Linux USB + wire LAN to your router.
Wait until it is online (IP address visible on the router management page, the imac will be named "grml"), probably 3-5min.
SSH from the Windows PC to the iMac and proceed with the PCIe check, then flash command (just follow the guideline in the post that come with the link to get the GRML files)
vBIOS from the courtesy of
@Ausdauersportler in the link below: My card was a Dell Red one, I picked the file with name "M6100-General-Enable.gop.rom.zip".
How to create an EFI boot screen vBIOS for Apple Late 2009 to Mid 2011 iMac systems using AMD MXM graphics cards - Ausdauersportler/IMAC-EFI-BOOT-SCREEN
github.com