I have two HDD,s and windows are installed on both. Obviously I boot from one of the HDD at any one time. I am interested to know whether the wear and tear of the HDD which is booted in, is greater or the same as the other HDD. -

If both drive are powered there would be little difference in the wear. The working drive would naturally get more wear due to head movement. This is not something to be concerned about. Most harddrives have an average life expectancy of at least 100,000 hrs. The key word being average. Any harddrive can fail at any time. Prudent computing practice dictates you maintain current backups of all needed files.

"Most harddrives have an average life expectancy of at least 100,000 hrs" I had to do the math to see what that works out to be...approx 11.4 years "And that's the fishing line, because Sharkboy said so!"

I got to wondering how manufacturer's are currently rating their drives. Western Digital states a 5 year warranty with 50,000 start/stop cycles. Seagate is the same. Hitachi Deskstar 500GB Reliability Error rate (non-recoverable) 1 in 10E14 Start/stops (at 40° C) 50,000 Availability4 24x7 MTBF4 1,000,000 hours MTBF = Mean Time Before Failure. That is One Million hours.

in my experience hard drive failure does not seem to correspond to how long it has been used. some last for years and years ,some fail much quicker. I have for years worked in large offices where all PCs are the same and stand on desks without being moved and are left on all of the time (logged off but not shut down at night). all hard discs same make/model, some last for years, some fail in less than a year.

cliffpage You are absolutely correct. That is why the above specs state Mean time, although in the case of Hitachi drives I am doubting that spec. They are just trying to look good due to past issues with IBM/Hitachi drive failures. I had 2 IBM Deskstars fail. The repacements were reconditioned and they didn't last long. I still won't buy a Hitachi drive because of that.