500 GB hard drive was slowly failing. I restored the system to an external HD. Then I used Disk Tools to erase the internal disk and write zeros; that apparently caused a fatal failure, so I knew the disk was bad. I used the ifixit repair guide for the EMC 2210, which is quite detailed.
Successful. I opened up the imac twice. First time, I replaced the HD successfully with a new 1 TB drive. About 3 hours from opening up to putting back the glass screen. ( I took time to vacuum and use canned air to remove the dust inside. I was relieved to see how easily the glass came off using your suction cups.) However, as the computer warmed up, the fans started running continuously at full speed. Using the Apple Hardware Test, I got the error code:
4SNS/1/40000000:T00P-132.000. It took some time for me to track down what this means. The "SNS" indicates a sensor problem. The T00P indicates that the thermal sensor attached to the optical disk drive isn't working properly. As the ODD is next to the HD, I may have disturbed its sensor. I took apart the Imac again, carefully checking connections inside along the way. I removed the small screws of the ODD, per your replacement guide for the ODD, and swung it out to see the thermal sensor assembly on the underside. I found that the sensor assembly had lifted partially from the ODD, so that the sensor itself was no longer resting against the ODD. Hence the absence of readings from it. Apparently, the sensor assembly had loosened up over the years (perhaps it had not been attached at the factory with adequate pressure). My replacing the HD, which involved untaping the thermal sensor wires from the chassis for both the HD and ODD, may have added another perturbation that pried the ODD sensor loose. Fortunately the glue on the ODD sensor bracket is thick enough and pliable that it was sufficient to press it back on the ODD, and it seems to be holding fine. I reassembled the imac (another ~3 hour start to finish, as I went carefully). Everything (HD and fans) works fine so far.
Your guides are very good and seem very accurate. On my imac I found that the Phillips #1 didn't remove the access door covering the memory slots. You need a #2, and that is what my "Everything Mac" manual indicates. But the center point of the screwdriver cannot be too long, or the Phillips blades will not gain purchase on the screw: its center depression is shallow. It would be helpful to have a full picture in your guides showing the inside after removing the LCD screen, indicating the relative placement of the drives, boards, power supply, etc. Here is one I took of mine.