My iMac is running REALLY slow - could my hard disc be dying?

Question

I have a 27" iMac with a Core i7 chip and 8GB of RAM.
It has always worked really fast with absolutely everything I have tried with it.
It worked fast in Snow Leopard. I upgraded it to Lion in September and it still worked fast (though a tiny bit slower perhaps than Snow Leopard).
In the last week, the machine's speed has slowed to an absolute crawl and I am almost constantly getting the spinning beach ball.
By a crawl, I mean that opening iTunes can take five minutes. Opening the Applications folder inside Finder can take five minutes. During times of this slowness, the spinning beach ball animates and I can move the mouse cursor, but I cannot get any response from the machine.
The machine is so slow that it is basically unuseable - I cannot wait five minutes to open a document!
What is really odd is that the computer will be really slow for 4-5 minutes, and then for perhaps 40 seconds or so I will get maybe one minute where things work at normal speed, before the speed drops back to a crawl.
Even restarting the computer takes an age.
My theory is that something in the computer has malfunctioned - either the motherboard or the hard disc.
I have run a few disc utilities (Disk Utility, Apple Hardware Diagnostic, TechTool 5), and the SMART status of the hard disc appears to be good and all reports check back successfully - although they all take longer than I'd expect. Whether that means the hard disc is really fine or not, I'm not clear.
I had one other hard disc die on me once, and that just died and was gone, but if my hard disc is dying then the problem it is showing is intermittent, i.e. it is only working part of the time.
At present I am making a backup of the iMac hard disc by having booted the iMac in Firewire Target Disk mode and manually copying files across. However, so far it has only managed 1GB of about 432GB and is estimating that it will take 2 days to complete the task.
Has anybody got any advice - is the hard disc or the motherboard a likely culprit?
The machine is under warranty at least, but I can't figure out whether to drag the machine into an Apple Store, or to phone AppleCare and get them to pick it up. If the machine would be repaired while I waited at the Apple Store then that would be great but I doubt this would happen.
Any advice?
P.S. I checked for runaway processes... nothing to find in Activity Monitor. CPU is about 5% max. Lots of free RAM. Hard disc is only half full.

Answer 1

mccltd wrote:
How do you load lion onto an exteranl disk and boot from it as I would like to give that a try.
I presume its an external usb drive?
thanks
Hi,
I used an external hard disc - Western Digital MyBook 500GB drive.
All you do is run the Lion installer, which you can redownload from the Mac App Store if you've bought it. As part of the Lion installer it asks which drive you want to install to.
To save having to redownload the Lion installer every time, once the download has completed from the Mac App Store, but before you have run the installation process, you can extract the disc image and retain it for future use (if you don't extract the disc image, then it gets deleted at the end of the installation process and you'd have to redownload it again next time).