Life of a drive is highly random, except for particular model batches that fail due to a defect (Fujitsu 20-Gig for example - all dead in a year and a half due to bad coolant formula inside one of the chips).

When my clients ask me, "How long do hard drives last?", my answer is always, "Around 10 years - but that doesn't mean yours won't be dead in ten minutes".

Now this is many years ago and may not be valid today, but a senior engineer at one of the manufacturers was asked, "Is it better for the hard disk to turn off the computer at night or let it run continuously?". He responded, "Every time you turn on a hard disk it takes about 8 hours off its life . . . ". Of course if you let it run all night it takes off about 8 hours of its life.

It is my understanding the heads no longer "fly", but are actually tail draggers, so wear is possible. On the other hand, in my experience, magnetic surface problems are very rare. Nearly all failures are electronic, and generally very sudden. "But it was working just fine yesterday."