Video Corruption after Clean Yosemite Install
Interesting problem here. We've got a really clean 27" iMac in the shop, it tested beautifully for all tests in AHT. Yosemite installed with no problems at all, smooth as silk.
All was well until Monday when we noticed corrupted video after boot up. What's strange is that video is perfect on Safe Boot or when booting from installation media, so it's definitely not hardware related. Creating a new user account also results in corrupted video so it's not a specific user setting either. We recently had similar graphic driver issues with a Mac Pro that was corrected by installing Nvidia's drivers for OSX. This iMac has an ATI Radeon HD 4850 PCIe graphics card and ATI doesn't offer drivers for OSX, so the same fix won't work for the iMac.
Any thoughts? Is there a driver for this card somewhere that I haven't found yet?