Hard disk was receiving predictive failures. Had to remove a SD Card from the SuperDrive that my kids crammed in there.
Opening up the iMac was a lot less stressful than I anticipated. Was prepared with the right tools.
If you replace the hard disk with a non-Apple hard disk, make sure you short the jumpers for the heat sensor plugged into the original drive (just connect a wire to the end of each jumper lead and seal it off with electrical tape) or the system fans will get very loud. Wash your hands before removing the screen cover and the screen. Keep a microfiber cloth and cleaner handy. You will get them full of fingerprints.