Apple iMac Graphic FW Update
Fixes issue that may cause iMacs to hang during startup.
Developer website: Apple
iMac Graphic FW Update fixes an issue that in rare cases may cause an iMac to hang during startup or waking from sleep.
Do not shut off the power on your iMac during this update. Loss of power could result in your iMac failing to start up.
Requirements
Mac OS X 10.6.7 or later