stonebro: Harddrives are about to die if;

- they start making metallic, scratching or beeping noises
- they are starting to read / write slower than usual and perhaps throwing up the odd corrupted file
- they consistently overheat

In the first case the drive is probably dead already. In the second case you should move to save your data asap but will likely be able to save most of it, and the drive will continue to function in some form for a good bit. The third example may lead to a physical defect which again instantly kills the drive, meaning you must either reduce the temperature or quickly back up your data.

The life expectancy of a standard mechanical HDD depends on use. Heavily used drives might not last for more than 2-3 years (this was the case with some of my shared drives when I was into that whole DC thing). I was uploading about a terabyte a week and it was basically a slow torturous death for my drives. More irregularly used drives should last 5 or more, but as with all hardware you can't really count on it.

Watch your temperatures and run diagnostics every once in a while. If you have really important stuff you need to put it in a mirrored RAID.