Wish I had found this site earlier.
This website for Mac Tech Support looks to be useful. Unfortunately, it is the first website I've found that warns that some of the higher speed ata hard drives they make now have a tendency to overheat in the iMacs. This really sucks because I just installed another high speed drive in the Mac. Hopefully, the third drive's the charm. Carla and I hadn't noticed any difference with the speed anyway.