Hello Dan, Just so i understand your amazingly thorough post, if I don't want to use the HD drive in my mid 2011 21.5 iMac, do I just follow the instruction provided, adding the SDD drive using the optical thermal sensor from OWC?? My confusion is that your link shows you how to add an SSD drive to make a dual drive with the HD, but when you mention the option of not using the original HD, you use the word "replace" and then mention the use of the OWC optical thermal sensor. Am I removing the old HD and replacing it with the new SSD or am I adding the new SSD keeping the old HD,but using just the SSD? hope that makes sense. thanks in advance.