HDD should be "consistently" between 30 - 55c. Above 55C and its starting to over cook its insides. Remember that heat = cooks = bad thing.
I don't understand why the newer iMacs are running hotter and hotter - compared to previous iMacs. Is it because "too much" stuff being packaged into a thin box? Is it because one's ambient temp is hotter then others? Or, it is because applications are doing "more and more" in the background - which is creating more internal heat? Or, is it because Loin and Flash don't like each other - which works the internal items harder (which creates more heat)? Perhaps all these reasons....
I don't believe in driving anything (car or computer) until it automatically shuts down. It would be like driving a vehicle until its reactive "too late" dash light comes on. "Too late" light because when it does come on (or when the iMac auto shuts down from heat over load), the internal damage is already done. If the iMac inside's feel hot, looks hot and smells hot, then guess what, its insides is hot. Ya, one of those "duck" things.
Here's the specs of my 21.5" iMac. re:
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/InternalTemps2010-08-30.png
http://i178.photobucket.com/albums/w251/Spike99-Pictures/New iMac system/FanSpeedJune25-2011.png
Notice its insides isn't hot (HDD is nice 43C - between 35 - 55 range) and its other temps are within cool range as well. All because I've tweaked its factory fan speeds and discovered what works best - for my iMac's usage. Yes. Sampling and "tweaking" its settings every over day for 4 weeks does allow its most optimal "cooling to fan RPM" best configuration truth float to to surface.
As a minimum, do purchase a small silent sound fan and install behind the iMac chassis. Something with physical speed control switch - to allow you to control its air flow. If blowing air against your iMac's back cover lowers its insides even a small 5C degrees, its well worth it. Especially if ambient temp in your computer room is higher then average as well...
Always remember that "heat = bad". Cool = good. And, cool = long term usage without breaking down and/or randomly hanging (from over cooked insides). Apply proactive cooling improvements and your iMac's insides will thank you... Just like your automobile likes to keep cool. And, your human body likes to keep cool as well....
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