My iMac hard disk has crashed twice, needing reformat, but appears to work after reinstall. Is this a hardware problem or normal?

Question

The hard disk in my late 2009 iMac has failed twice in the same way. The symptoms have been:
* First, the system starts to "stutter", freezing up at random intervals for some seconds.
* This goes on for days or weeks, increasing in severity until I verify the hard drive
* The disk check shows the disk is in need of repair, and the repair attempt requires me to boot from install DVD
* When I repair the disk from the tool on the Install DVD, the repair fails and the disk is no longer usable
* Reformat, reinstall and restore from Time Machine and everything works fine again
This happened once in end 2011, and now again recently. Is this something that is normal, or an indication that the hard drive is failing in some subtle way?
If the latter, would it make sense to replace it or will that cost me more than the value of the iMac at this point? As I understand this is not something for the fainthearted to attempt on their own.

Answer 1

Could be that issue with a cable not reporting the thermal
sensor status, as equipped in the original hard disk drive...
•Proprietary Cable can put brakes on upgrading Late 2009 iMacs - OWC blog:
http://blog.macsales.com/2751-proprietary-cable-can-put-the-brakes-on-upgrading- late-09-imacs
•Diagnosing 2009 & 2010 iMac Fan Speed issues after upgrading the Main hard drive:
http://blog.macsales.com/19617-diagnosing-2009-2010-imac-fan-speed-issues-after- upgrading-the-main-hard-drive
Thus the local recommendation to replace the HDD with
one of the same brand & model with internal sensor; &
often with a service if this type, the original cables are
inspected and replaced. Sometimes in the process of
disassembly the originals can be messed up, too.
{iStat pro widget? handy if you can find it now}
But the sensor is inside the hard disk drive.
Your SSD has none. So the computer may
be overheating without feedback to run fans
at a speed matching the demands of CPUs.
If the iMac overheats (or is led to assume so) it will shut off.
A possible work-around is suggested, but may not work.
Matching parts with the model is important.
Hopefully this helps...
Good luck & happy computing!