Is there a video site like YouTube that shows how can this be done. I tried searching for it on youtube but there were only old videos of the iMac 24inch HD install...for replacement. Nothing on how to add another HD for the iMacs 21 or 27 inch.
Do you mean adding a SECOND HD or just replacing the old one? Here is tutorial how to replace the SuperDrive with SSD, there is no space for another 3.5" HD I think