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Apple today released iMac Graphic FW Update 2.0, a firmware update to addresses graphics issues on the company's latest iMac models.About iMac Graphic FW Update 2.0



From MacRumors:
This firmware update fixes an issue that in rare cases may cause an iMac to hang during startup or waking from sleep.
  Apple Addresses Graphics Issues on Mid 2011 iMacs With Firmware Update