thank's to Mr. Backlin in the Apple Community for his helpfull tips, installing W8.1 in EFI-Mode on my iMac 2011. Couldn't install the update from W8 to W8.1 before without the known graphic driver issues at the end of the setup. So I followed his tips: https://discussions.apple.com/message/23604163#23604163#23604163 now I have successfully installed W.8.1 on my iMac, but couldn't get the right AMD driver. Tried latest Catalyst driver, Beta driver from AMD's website, tried the driver-check-program. All failed after extracting at the beginning of the installation. has anyone found the right driver for my Radeon 6770? I think, it's a mobility graphic card, so has anyone a link to a working driver? Thank's!