Here's an excellent article on upgrading your 27" 2009. It also has links to compatible drives: http://www.everymac.com/systems/apple/im...

UPDATE

The problem with changing drives is the thermal sensor. If you stay with the same "brand" of drive there should be no problem. If you change brands, you will need to change to that companies sensor. As far as I know there are three different ones, the Western Digital 922-9225, the Seagate 922-9224 and the Hitachi 922-9223

I tried replacing the 500GB Seagate drive in my late 2009 21.5" iMac, with a 1.5TB drive, also from Seagate. But I had the same problem with the fan slowly reaching full speed. So I had to swap back the 500GB drive for the time being.

At least with Seagate drives, the connector for the thermal sensor cable is normally used as a jumper block for speed selection. See page 20 of this: http://www.seagate.com/staticfiles/suppo...

Apple-branded Seagate drives have custom firmware to change the function of the jumper block.

To install a non-Apple branded drive, you need an external sensor cable, the same type used in iMac prior to October 2009. I have ordered P/N 922-9214 for my mac and will install it together with my new drive once I receive it.

UPDATE!

3 days ago I changed my '''1TB western digital cavier Black hard drive on my 27" 2.8GHz I7 iMac to a 2TB western digital cavier Black hard drive. With the instructions listed here in the site it was actually pretty easy to do. You must take care when pulling and re-installing wires. I almost found out the hard way but all is good. On to the fix;

I installed the 2TB hard drive and put the display back on with 2 screws and tested. everything went well and the computer booted. I installed the new system and ran the hardware test, it failed and the fans started blowing. I looked at the instructions and double checked everything. I found that I put the thermal cable on the wrong side. The instructions stated to put the cable closes to the sata cables. I did that and put the screen back on with 2 screws and booted the system. I ran the hardware test again and failed. I tried zapping the pram-zram-bram the whole alphabet ram, and nothing worked. I must have tried the hardware test 4 times after doing some trouble shooting.

I took the computer apart again and I took the thermal cable and turned it a half a turn with the grey cable now facing up. I inserted the plug, put the screen back on and ran the hardware test and it passed!!! YES

I put it all back together installed all my software, final cut pro, photoshop CS5 and so on and everything is working fast and great.

Hope this helps.

Rick.....

I received the 922-9214 cable today and have replaced the 500GB Seagate drive the iMac came with, with a Seagate 1.5TB 7200.12 disk. Using this, the fan speed is stable. I also ran AHT twice without errors.

So to conclude: Apple-branded Seagate drives are different from regular drives from Seagate. Swapping out a Seagate drive bundled with the iMac, for a bigger drive of the same brand will not work, unless you also replace the temperature sensor cable with the type used for affixing to the body of the drive, i.e. exactly the same cable used for the optical drive in the iMac.

The part number above is for a 21.5" late 2009 iMac. You may need a different P/N if you have a different model of the iMac.

Hey! Zakipj, thank you for the reply!! Please keep me posted as to you results, I'm VERY interested to hear how this latest fix turns out. Jeff

That's exactly the answer that I was looking for!! Thank you!!!! now on to getting that optic drive heat sensor for my late 2009 27" iMac. Thanks again!! Jeff