Is turning off hard disks harmful

I set Windows 7 to turn off hard disks after 20 minutes of nonuse, which it does a really good job at – it seems to turn off individual hard disks that have been idle for 20 minutes exactly. When I access a drive that I haven't used in 20 minutes, I can hear it firing up and it takes a couple of seconds to access it.

Now Windows is turning off and turning on my hard disks several times a day.

Can this on/offing be bad for the drives? The case is small and I'm trying to protect the drives from overheating really (they get VERY hot!).

Edit: The accepted answer doesn't mention this explicitly, but from Minimizing hard disk drive failure

Power cycling control

Shutting down and rebooting a computer or resuming it from hibernation cycles the power to the drives in the computer. The spin-up operation performed by a drive after a power cycle is believed to place more stress on the drive than running the drive continuously for a long period of time.

Based on professional experience of system administrators, it is believed that there is a direct relationship between the number of power cycles of a computer and the probability of failure of its drives**. In other words, a computer with a high uptime may have a lower probability of drive failure than one that has its power cycled routinely.

** Attribute #4 of S.M.A.R.T. is Start/Stop Count, which seems to indicate that start/stop count DOES play a role in disk fitness and when to expect a failure.

See also

• Thermal stress relief with Power management

… Energy Star compliance results in an increase in daily power up/down cycles. The consequent thermal shock and mechanical stresses on the system can adversely affect its life…

• Computers and Monitors – When Should I turn them off?