common cause of hard drive failure.. Heat..

Test:
run system for one hour.. be sure to be oing some hdd intensive work.. virus scan and or a defrag..

after the hour .. remove the towers outer case/side.. place index finger tip on the top metal surface of the HDD.. and press firmly..for a slow count to 10 if the surface is warn to very warm.. you will have very few problems.
If on the other hand it is extreemly hot and you are unable keep your finger there for the full 10 seconds with out the distinct aroma of burning meat.. you HDD and any replacements will die after a short life..
Also If the PC is in an area subjected to lots of vibration..ie ithe tower is sitting on top of a 200w sub-woofer, .. you also will encounter hdd failure problems..

It is always good form to have your data backed up.. regardles how well things are running..

BTW : yourdeadin.. if your dead hdd's haven't lived in the types of enviroments mentioned above. send me a PM.. I will take the bodies off your hands.. the drives that I have rejuvinated that had not been subject to those enviroments or high humidity and did not have a controller card problem ( - they die with no hint)