I’ve always wondered how long a hard disk’s life is if you left it on 24 hours a day (which I do with some of my server computers).
Well on average, according to this site,
The average service life of a hard disk is usually given as “5 years or 20,000 hours.” This means if a hard disk operates 24 hours a day, you will need to replace your drive with a new one every 1.5 to 2 years. (For specification details, refer to your hard disk drive’s user manual.)
The measure of service life used by most hard disk manufacturers is “mean time between failures” (MTBF)*1 (usually quoted at between 300,000 to 500,000 hours). The ratings (for expected service life) used by PC makers are more conservative, as they are based more closely on “real world” usage conditions. Since expected service life and MTBF figures can differ by hundreds of thousands of hours, we recommend you use the expected service life value. For example, the usual service life of a 2.5 inch drive will be almost 5 years or 20,000 hours, if used at the temperatures specified for your PL. However, if a drive is operated continuously, i.e. 24 hours a day, its service life will be shorter.
Also, there’s a way to put your hard disk under sleep mode on most operating systems, I suggest you to turn that on:
I see no reasons why you should leave your hard disks on if you have left your computer for couple minutes or even hours.
For Windows operating systems, go to ScreenSaver->Power then set “Turn off hard disks” to “After 3 mins” so you save both power and your hard disk life. Also digitgeek.com has a great article on maximizing more life out of your hard drive.