Symptoms of Hard Disk failure

You can use Apple Hardware Test. If it reports an error it is certain to exist. If it does not, a problem may still exist but it must be diagnosed by describing its symptoms.

Other than AHT Disk Utility is the best option. Don't use third party utilities, they often report errors where none are present, and may even cause them to occur.

Do you believe your Mac is overheating and shutting down on its own? There could be a number of reasons for that. AHT may help find the cause and perhaps provide a clue to what needs to be fixed. Whatever the reason, you should back up your hard disk's contents in the event you can't get it to boot at all. If that becomes impossible, there are still ways of retrieving its contents but the methods become progressively difficult.

You need to find your original grey Applications Install DVD or whichever one shipped with your Mac that has the words "Apple Hardware Test" or "AHT Version xxx" printed on the disc. Restart your Mac with that disc inserted, while holding the d key. The KB article above explains the procedure.

A copy of AHT was supposed to be installed on your hard disk when upgrading to Lion, but this failed to install on some Macs that were upgraded to Lion. Loading AHT from the disc is certain to work.

That error indicates a problem with the HD or a problem communicating with it. Consider it good news in that you found it before it fails completely. Check here to be sure you are not affected by the Seagate recall:

http://www.apple.com/support/imac-harddrive/

I do not recommend TechTool Deluxe. Broken hard disks can't be fixed. Fortunately they are not unreasonably expensive to replace. The Apple Store can do it, but you can do it yourself if you are sufficiently adept at such things.

If your hard disk is a Seagate, and you purchased your iMac between October 2009 and July 2011, definitely bring it to the Apple Store even if your serial number shows that yours is unaffected by the recall.

Back up your hard disk before bringing your iMac anywhere near an Apple Store. Failed hard disks replaced under the recall will not be returned to you.

Genius Bar Reservations: http://www.apple.com/retail/geniusbar/

That's easy enough - click the  menu > About This Mac...

Click the text - where it says "Version" - right below the bold OS X twice, and your system serial number will appear.

iFixit.com has detailed pictures and instructions for replacing iMac hard disks. Except for the just released iMac models, removal of the front glass is easy, keeping dust from getting caught behind it is not.

HunterBD wrote:

What I want to know is where - inside my iMac - is the SN stored. Presumably it cannot be on the hard drive as it would be 'lost' if a hard drive is changed.

The SN is burned into firmware using a procedure available only to Apple, and yes it will survive a hard disk transplant. It should also match the one etched on the bottom of its stand. There may be other places they don't tell us about.

HunterBD wrote:

I've now read many threads in the Apple Support Communities looking for a list of symptoms likely to be experienced if a hard disk is heading towards total failure, but haven't so far managed to find such a list (just lots of snippets of info!).

Generally the reasons of hard disk failure and their symptoms are always same, it doesn't matter what operating system you're using, be it Mac or Windows. I can share this blog post here in which you can find the 5 most common reasons - Top 5 Reasons for Hard Disk Failure

I hope the shared blog post would be helpful here.

Regards.

HunterBD wrote:

Secondly, is there some way (other than the Apple Disk Utility) in which I can check my WD drive whist it is still within my iMac? Any advice will be much appreciated.

Does it help - http://support.wdc.com/knowledgebase/answer.aspx?ID=14925

Thanks.