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.