From Apple Insider "A rare weekend software update promises to cure system freezing issues in early 2009 iMacs equipped with an ATI Radeon HD 4850 graphics chipset. Released on Saturday, iMac EFI Firmware Update 1.4 (1.7MB) specifically addresses the hard lock-ups that occur at random and leave iMac users with the custom-order graphics with no choice but to reboot their systems to regain a usable computer, regardless of whether they're using Mac OS X or running Windows in Boot Camp. It also mends problems owners have encountered which prevent the systems from waking properly from sleep when using Boot Camp, Apple says. The update requires Mac OS X 10.5.6 or later. The company hasn't explained what has caused the problems, which have triggered lengthy discussions on Apple's support forums and, though producing different symptoms, have drawn parallels with an earlier ATI graphics freezing issue from 2007."