Test Criteria - Temperature
As already mentioned above: Increasing rotation speed entails higher friction, which in turn leads to higher heat dissipation. Drives running at 10,000 rpm (SCSI only) necessarily need active cooling, as the magnetic discs can lose the data integrity at increased temperatures. IDE drives are meant to run without cooling, as the expense factor is more important than with SCSI models. Taking a look at the temperature numbers of the test drives, you can run all the hard drives in this review without active cooling. However, it is always an investment for data safety to use a hard disc cooling system. It can even become a performance issue, as most drives will regularly recalibrate the heads at high temperatures. That may be a small argument in favor of expensive SCSI drives: Everybody who needs continuous data streams (such as with hard disc recording or video editing) will be grateful if the drives only recalibrate when it's absolutely necessary. That's in fact an issue which is usually considered during the development of high end drives.
Please remember that there are other components, which suffer from high temperatures as well. Every drive comes with its own electronics (IDE means integrated disc electronics), featuring at least a controller chip, a firmware chip and maybe one or several cache memory chips. High temperatures also easily affect those chips. As you already know from processors, the chip temperature may not exceed a certain value. Of course there is hardly a way of measuring the real chip temperature, so the common option is to use the surface temperature. Most Pentium III CPUs are specified for up to 80 degrees Celsius, for example. Improper cooling results in even higher temperatures, which may result in turn with a dead chip. The same applies for hard disc components of course.
Test Criteria - What's too hot?
You will probably not be able to determine when the drive is running too hot, but you can easily find out if the drive temperature exceeds approximately 50 degrees Celsius. Run your computer for some time (at least two hours). After this time, you should carefully open the case and put your hands on the disc drive (please don't touch any electronic part on the bottom). It will be best to keep the computer running, as turning it off will cool down most devices pretty fast - at least out of potential critical temperatures.
A direct contact of hot objects to your skin causes pain at approximately 55 degrees Celsius. That's what you should also try to find out with your hard drive. If you feel that you have to take away your hand from the disc's surface, you can be sure that the temperature exceeds approximately 50 degrees. In this case, I would consider getting a hard drive cooling system to ensure a long lifetime for your disc.