27-inch iMac Graphics Firmware Update 1.0
Updates the graphics firmware on ATI Radeon HD 4670 and 4850 graphics cards to address issues that may cause image corruption or display flickering.
The iMac Graphics Firmware Update will update the graphics firmware on your iMac.
Do not disturb or shut off the power on your iMac during this update. Loss of power could result in your iMac failing to start up.
For detailed information on this update, please visit this website: http://support.apple.com/kb/TS3181