Always-On HDD?

The bearings in your hard drive, the electronics on the drive and the electronics in the enclosure will all be exposed to less heat and thus will last longer if you turn the drive off when it is not in use. The chances are that your drive will work happily for at least the next three years (which is the warranty period for quality disk mechanisms) but why take a risk? You'll also save on the energy consumption of a disk that is not in use. It'll be just a few dollars a year but get in the habit anyway. When you end up with loads of computers, hifi amplifiers etc, you start to notice the financial benefit of turning stuff off.

When you first start a drive, the disk head *may* cause wear to the disk platter, but in reality this wear doesn't cause early disk failure. Just make sure that you aren't switching a computer on/off more than a couple of times of times a day. Given that the sleep mode in modern computers spins the disk down, there is no difference in wear behaviour between waking from sleep and a cold start.

At work where we have sufficient PCs to perform meaningful observations, the disk failure rate does not appear to be related to how frequently the PCs are cleanly shut down. The increased failure rate after a building suffers a mains power failure is noticeable, however. The worst environmental problem that I have observed was when the bearings on an air conditioning chiller started to fail, feeding a machine room with microscopic metallic particles in the chilled air for several months. Servers failed in unusual ways for two years after the replacement of the chiller.

Phil