With Yosemite and the SMBIOS set to Mac Pro 6.1 I was getting the same with an Nvidia GTX 780. The machine continued to function, just the screens went black. I found this somewhere (can't remember at the moment) but had it written down as it solved the problem. You need to connect in your host and to make modification of the Info.plist file which is on the way /System/Library/Extensions/AppleGraphicsControl.kext/Contents/PlugIns/AppleGraphicsDevicePolicy.kext/Contents/ In this file you look for Mac-F60DEB81FF30ACF6 = Config1 line and you replace it with Mac-F60DEB81FF30ACF6 = none Save. You do update cache, for example by Kext Utility. Reboot Hope it works for you too.