I have an iMac 27" mid-2011 with a semi-dead 6970M, 1gb GPU - I only got the system to boot by disabling the GPU kernel extensions(moving /System/Library/Extensions/AMD* to a backup location = weird and slow graphics, but better than just a gray screen) and Apple has refused to fix the problem because it's more than four years old - even though it's a recognized issue.

So, I need a new card.

• Are all MXM 3.0 B cards compatible or does the card need Apple firmware?
• Would a 6970M card for a Dell Alienware laptop or another card with the same chip as one of the iMac 2011 models work?
• Where can I get a card that works? (<400$)

I've see a lot of questions and answers that dance around these questions but none that actually answers them specifically so ANY help will be greatly appreciated.

• ''Are all MXM 3.0 B cards compatible or does the card need Apple firmware?"
• "Would a 6970M card for a Dell Alienware laptop or another card with the same chip as one of the iMac 2011 models work?'

I found out Apple uses special firmware and for a card to work it needs an EFI flash with Apple firmware.

A card without Apple firmware will simply result in a black screen.

A friend of mine did suggest I could remove the BIOS/EFI-chip from the original board and reflow it onto a cheaper card - but that will probably be too complicated for a lot of people and I already had a new card on the way.

Cards that already have Apple firmware (pulled from iMacs) are interchangeable though - I upgraded from the 1GB 6970M to the 2GB version.

• "Where can I get a card that works? (<400$)"

You can get a working card on Ebay or Aliexpress - any MXM 3.0 card pulled from an iMac should be fine. I paid ~300$.

Lastly, as @mayer pointed out, if you have an iMac 27" Mid 2011 with issues, the GPU is most likely the cause and Apple will fix it if it's no older than four years, because it's a recognized issue.

Apple refused to fix this one because it was sold in 2011 but if you want to fight Apple, maybe that'll work too - I chose to fix it myself instead.