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If it feels hot, if it looks hot and if it smells hot, then guess what? It is hot. Unfortunatley, the iMacs and their small slim all-in-one box design run hot. It's a side effect of their compact design.
Any internal HDD above 60C is getting too hot. The longer it remains in the 60+ heat range, the higher risk of HDD problems. If machine doesn't auto shut down until 100C, then major problems with be forced onto its internal HDD. It would be like driving a vehicle "way past" its over heat light comes on state. Very, very bad to continue to "over heat" within its too late heat range.
Suggest installing Islayer IStats Nano (which is a free widget) and using this tool, ensure its internal fans are coming on. And, they are changing RPMs based on different heat ranges. If its internal HDD is above 60C, then its over heating. Over heating before it does an auto-shut down.
For "low cost" proactive cooling methods, remove its memory simms install cover (its under the sceen and its opening cannot be seen anyway), install a 6" "silent running" fan behind the iMac screen and install ISlayer iStats Menu 3 (and increase its fan speeds). Take a "proactive" approach in applying better cooling to your new iMac - before over heats.
If wondering, my iMac heat and fan RPM details are:
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/FanSpeedJune25-2011.png
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/InternalTemps2010-08-30.png
For more details about iStats Menu and its ability to control heat, surf:
http://www.tuaw.com/2010/04/27/a-hands-on-look-at-istat-menus-3/
http://bjango.com/mac/istatmenus/
Hope this helps in your rearch...
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