As 7200rpm hard drives are the standard today, and high-rpm drives are found in most desktop PCs, and not just in servers, hard drive cooling has become a more important issue
Most recent IDE hard drives come with internal temperature sensors, and are capable of reporting their operating temperature via SMART; software utilities are available for displaying the temperature of your hard drive. This temperature readout is not very precise, but definitely good enough to give you a hint whether your drive is running too hot or not. Consult the website of your hard drive manufacturer to learn about the maximum operating temperature of your hard drive. Typical maximum temperature range is 50-55°C, with some drives being able to operate at up to 60°C.
There is, however, an important factor to consider:
Unlike CPUs, which are reliable parts, and fail rarely (even when operated very close to their maximum temp, hard drives tend to fail more often than any other part in the PC. The MTBF (mean time between failure) of a hard drive can be substantially increased by lowering the drive's temperature. Some drive manufacturers even state this fact in their drives data sheets; but even if this information isn't included there, it is still true. So, even if your drive never exceeds its maximum allowed operating temperature, additional cooling will improve the drive's reliability. Unless you do daily backups, a hard drive failure usually results in substantial data loss - investing a small amount of time and money in your drive's cooling solution can often prevent the need for data recovery (which isn't cheap).
With optical disks, overheating usually results in destruction of the surface that actually contains the data (e.g. chemical dye in the case of CD-R/DVD-R media). However, with hard drives, overheating typically causes long-term failure of the mechanical and electrical parts of the drive; the magnetic platters that carry the data are less affected. Therefore, data recovery companies will normally be able to recover data from damaged overheated drives - but be prepared to pay much more for getting your data back than for a new drive
Does Hard drive cooling affect performance ?
Old hard drives had a thermal recalibration loop - from time to time, the hard drive would make a short pause and recalibrate itself. When the drive warmed up, the platters expand, and their geometry changes, thus making recalibration necessary. With such drives, good cooling resulted in less frequent recalibration, and therefore did improve performance, especially for Audio/Video applications. However, these days are long over - all recent drives with embedded servo do not have a thermal recalibration loop. Temperature will not affect performance in any way.
Do drives only get hot when accessed often ?
No. What generates most heat in a drive is the friction between the platters and the air that surrounds them. The magnetic plates inside the drive rotate at a high speed, but the air inside the drive doesn't, and even if the plate is perfectly flat, there's a lot of air resistance, so a powerful spindle motor is required to keep the drive spinning at high speed
The movement of the drive's heads and the heat emitted by the drive's electronics only have a small contribution to the total heat emitted by the drive. Therefore, even drives that are mostly idle still require cooling (unless, of course, auto-spindown is enabled and the drive shuts itself down after not being accessed at all for a longer period of time).
Hard drive cooling solutions
The best solution for cooling hard drive is one that is well-integrated with the case cooling concept. When purchasing a case, make sure that fan intakes and drive bays are located in a way that proper cooling of the drives is possible. A fan bay for a large fan (the bigger, the better - server cases usually come with 120mm fan bays) right in front of the drive bays is perfect. If your case has a fan bay suitable for cooling drives, get a good fan and install it there. This is cheaper than buying a dedicated hard drive cooler, provides good cooling for the drive, and will have a positive effect on overall case ventilation.
Dedicated hard drive coolers
If your case does not allow proper ventilation of the drives by using normal case fans, you might want to look at special hard drive coolers. A multitude of different models are available on the market; they usually come with small 40mm fans, which are less efficient and typically louder than larger case fans. Some come with heatsinks for hard drive cooling, others rely on fans alone.
The most simple hard drive cooler simply consists of a 5.25 drive bay cover with two embedded fans.
Noiseblocker - NB drive Gladiator HDD Cooler for all 3.5" Hard drives 120mm Cooling Fan 17 dB/A - Fits into a 5.25" Spare bay drive
Revoltec Dual Fan Hard Drive Freezer Aluminium Silent 20dBA 3 colours to choose 3.5" HDD cooler
Vantec - 3.5 Inch Harddrive Cooler Dual Fan Twin 60mm fans - high airflow (28CFM)
Xilence Icebox HDD Cooler with 4 Heatpipes
Zalman ZM-2HC2 Heatpipe cooler
Silentmaxx HD Insulation Rev 2.0 aluminium