Quote of the day: "The failure rate of hard drives is 100% so it's not if your hard drive will fail, but when."
from www.logmein.com encouraging people to use their backup service.
If that fact doesn't scare you into making regular backups I don't know what will. Of course, 'when' your hard drive fails could be many many many years away but all mechanical things have a limited life and will wear out. MTBF is often given on hard disks and that acronym is "Mean Time Between Failure". I'm sure you all know that "Mean" is a mathematical term for a type of average. You can see more how it's calculated courtesy of Samsung. So, how long is the typical hard drive expected to last? One of samsung's models give 500,000 hours which is equal to 57 years of continuous use. Some will last less time than that, some will last longer. Somehow, I think the 'mean' drive will be obsolete even to a hoarder and computer recycler like myself.
Just for interest, I've had 4 hard disks fail since my first PC in 1994. One was brand new (first PC, just my luck!), one was a laptop drive and two were from different servers (both part of a RAID array which meant no data was lost - phew!). In the distant past I used to backup all our computers to tape drives. Today I send backups automatically every night to an off site server full of hard disks. Occasionally I restore some files and check they are still working. If you haven't backed up for a while - how confident are you your disk will last another 50something years...
PS - I chose Samsung as the example MTBF simply because their site came up first on a Google search, here at work we have a number of different manufacturers hard disks in our machines and I have no preference between any of the disk manufacturers.