The most affected is the Radeon HD6000 series in the 2011 models, then the Radeon HD5000 in the 2010 models and finally the Radeon HD4000 in the 2009 models. However, Apple's EFI does not support the VBIOS used on the "non-Apple" cards, some people thankfully figured out how to patch the VBIOS to restore EFI video output support as well as other features (native brightness adjustment, external display support) on some cards. If you are considering replacing the GPU in your iMac, you should REALLY read this post as it is appears to be the central repository for all things related to iMac GPU replacements - including more information on supported cards and available VBIOS: https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/