To cut the long story short – clean all the air vents on your iMac to avoid overheating. There are three of them at the bottom of the casing. After close to two years of uninterrupted use my iMac started to to run a little hot. Especially so when doing some intensive photo processing in Lightroom. At first I blamed it on a hot weather we have here in Switzerland. Then I thought it might be caused by new features in Lightroom 3 (lens profiles). But the situation was getting progressively worse. To the point that I could no longer export any photos. CPU was reaching 80 degrees centigrade, HDD bay was getting towards 70. I was getting more and more warnings from the temperature monitor. Then it dawned on me that I have not done any cleaning since I bought it. All it took was a simple swipe with my hand to realize how much dust has accumulated on the vents. After the thorough cleaning with the vacuum cleaner all is back to normal. Exported close to four hundred shots in one go and the CPU has not gone over sixty five degrees. One more vent is to be found just over the mains plug. Do not forget about that one. I think it must be for the CPU but I am not sure.
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Without wax,
yOOrek