My hard drives are in a 4-3 HDD enclosure bays, which a 80mm fan per enclosure propels air at about 1200rpm to keep the drives cool. The enclosure is built from aluminum and have a tray per hard drive. I also have 4x120 1000rpm case fans, two blowing air in and two blowing air out.

On a summer day (room temperature without air-con would be at 28C -  32C), the hard drives run from 37C, up to 40C, the highest it has exceeded was 43C (The most it I've run it on this temperature would be no more than an hour or so for once a week in this season. Once I have the air-con on, it'll sit back to 35 to 37C again, with no higher than 39C. I have standard Seagate drives. Are these good temperatures that my drives are running at? I have been told not to have them run past 40C, others say it is fine to have drives run past 60C as they are built to run at 60C+, which I find to be harsh, nor would I would like to have them running at.

I have only Seagate drives.  I aim to keep them below 35c.  They are 26c when spun down and typically 30-31c when checking parity.  But in the summer I expect that to go up to the 35c when checking parity.  I have a 120mm fan in front of them.  I built a sort of wind cage around the 7 drive bays with cardboard and tape (looks better than it sounds) so all the air from the fan blows through the drives.  I block off the empty slots.  The fan is a 2000 rpm 64CFM running about 60% speed right now (not much really) and that is all it takes to keep those temperatures.  

 

Yes, hard drives can withstand higher temperature but they will last longer and be more trouble free if you keep them cool.  Laptop drives are designed to run at higher temps but my laptop harddrives have always died within 3 years.  Even if you have your machine in a closet you should vent the heat under the door.  CPU's and other chipsets can withstand heat much better than harddrives can.

 

Hard drives aren't built to handle 60c (140F).  That's the maximum temperature (for how long?) before the manufacturer thinks they will fail at any moment.    So they work in the Iraq desert for 6 months tops.  

As an example, the Western Digital Greens are rated between 0 and 60 while in operation. But who knows how long they will last at that temperature.

 

Also, I had three drives in an unraid testing server with no ventilation running at temperatures between 60 and 65 for a couple of weeks. They are all still working fine except one has a loud bearring whine which was probably caused by that. The temperature sure kept me watching and nervous. I would personally say don't go above 50 and you should be fine. Or just think, the lower you keep their temperature, the longer you can expect them to last.

As an example, the Western Digital Greens are rated between 0 and 60 while in operation. But who knows how long they will last at that temperature.

 

Also, I had three drives in an unraid testing server with no ventilation running at temperatures between 60 and 65 for a couple of weeks. They are all still working fine except one has a loud bearring whine which was probably caused by that. The temperature sure kept me watching and nervous. I would personally say don't go above 50 and you should be fine. Or just think, the lower you keep their temperature, the longer you can expect them to last.

Most will advise not going over 40C.  arrange for more airflow across the drives if temps go above that.

I have one seagate drive that runs around 37C with normal operation and with it running all the time for rtorrent.  When doing a parity check it can get up to about 43C but not usually any higher.  Since it does not run at the 43C constantly I don't bother to make it run any cooler.

 

My suggesting it to try and keep the drives below 40C for normal operation and below 45-50C for parity checks.  I covered a lot of vents on the side of my case so that air has to flow in over the drives and then out the back.