You hear a clicking sound from your computer. Shrieking, you see error after error pop up, and then nothing.
Everything is gone. Your documents, your music, your programs, all gone.
... and then some tech geek turns around and tells you that you should have backed things up before everything died. But how would you know when it is dying?
Oddly enough, your hard drive actually does inform your system (most times) when something is happening. Believe it or not, your hard drive isn't conspiring to die at the exact worst time. :P
There are programs that exist that monitor statistics your hard drive sends to your machine through what is called S.M.A.R.T. (Self-Monitoring, Analysis and Reporting Technology). While some of these statistics are specific to your hard drive maker, most times the important ones are there.
One of these statistics, and probably one of the more important ones, is called Reallocated sectors count. In short, your hard drive is broken up into many sectors - tiny sections of the hard drive data is stored in. These can go bad. In fact, on old hard drives it wasn't all that unusual to see so-called Bad Spots appear all around the drive.
Hard Drive manufacturers got smart though - instead of actually making the hard drive more durable, they just added some spare (and hidden) sectors. These spare sectors are swapped in for the bad ones. This counter tells you how many times it has had to do that. Higher numbers mean more of the spare sectors are still available. Lower numbers, fewer. Some manufacturers start this count at 100, some at 200. Why, I have no clue.
If you see your hard drive go through a lot of these, there may be something causing physical damage to the hard drive and you should back it up and replace it immediately.
I'm not going to go in depth as to what each of these statistics mean. Even I'm not 100% sure on a lot of them. However, there are programs that will try to approximately when it will die. A lot of times, they are wrong. HDD Health, a program for Windows, would tell me that all of my hard drives are supposed to die June 12th, 2005. They better not. Of course, as soon as I removed one of the hard drives that I know is on its way out, it bumped up the Date of Death to 2008... yeah... but it'll also pop up telling you when something major has changed.
Well, I hope this helped!