iMac inconsistenly hanging and showing beach ball

Your own advice is fairly good in this scenario. There is a possibility that you have just some serious file system corruption, but usually Disk Utility should catch that. If you want, you can boot into single user mode (CMD S on boot) and it will give you a little file system cleanup command you can run fsck -fy and then press enter, when it finishes you reboot.

If this doesn't help, reinstalling after performing a backup is not a bad idea because it will isolate it to a software or hardware issue. The hard drive going over some area repeatedly could be just file system screw ups, but usually repeated noises are a bad sign for a hard drive, clicking etc.

If the problem comes back after the reinstall you definitely want to look into a new hard drive. Four years is not an uncommon life span for some hard drives.

Also just a quick point of clarification, the S.M.A.R.T status doesn't predict all failures, it just presages specific ones it can determine. Just because it isn't telling you your drive is failing doesn't mean it wont. Keeping everything backed up is a really good idea at this point.

I'm pretty convinced that this is a case of a dying drive. All the signs are there.

The best thing you can do is get as much data off as possible and then get your machine repaired at an Apple store or authorised service provider. Unfortunately a HD swap on an alu iMac is neither entirely trivial or fun.

During the backup process I'd recommend not using the computer for anything else and closing all unnecessary apps and background apps/services.

You are clearly aware of the issues involved and potential solutions. For the safety of your data I would recommend 2 additional things you can do.

A disk which makes unexpected noises is probably failing. The failure may happen soon or take some time. Nevertheless, it's always best to assume the worst, to check you backups work and to get a replacement as soon as possible. S.M.A.R.T. is useful although a random check may not show a problem. For continual monitoring and visible notifications of S.M.A.R.T. status, you should consider installing a utility such as SMARTReporter.

Memory, especially cheaper memory, isn't always reliable. Checking for memory problems is best done when you first get a new computer, and whenever you upgrade or install new memory. Of course, with the issues you describe, this is also a good time to investigate further. The most common method is to use one of the available memtest applications. There are commercial and free versions available. Free versions include:

• http://pyropus.ca/software/memtester/ although you may have to build the application yourself, otherwise download a pre-build version. I have used this to check newly-installed memory.
• http://www.memtest86.com/ offers a free download although you need to create a CD from this and boot from it to use the application.

and for a cheap, commercial alternative:

• http://www.memtestosx.org/joomla/index.php

In addition to these, the Apple Hardware Test CD or DVD that comes with a new machine also has memory test options.

I think this issue was due to a faulty memory DIMM. The original 1gig that came with the computer was starting to give in, I determined this by first removing the 2gig DIMM (3rd party memory installed). The computer then worked for a few hours without any hassles, but the hanging started up again after I did some stuff on it. I then switched the 1gig for the 3rd party 2gig, and then started up without any problems or hangs. I presume the 1gig was starting to give in and that was what was causing all the beach balls and hangs.

Thanks for all the suggestions. But I hope this was the issue, as I'm no longer having any issues.