I did an up grade to Yosemite and my old Mac Book Pro isn't happy. It qualified for the upgrade, but wasn't happy with my meager 8g of mem. I then used Carbon Copy and reinstalled Lion 10.7.6 which is fine with my Mac Book. What I would really like to do is wipe my HD and do a clean install of Mountain Lion 10.8 which I have ordered from Apple. Question….. Is the 10.8 that I will receive from Apple via e-mail do a compete install from an external disk? In other words I'll download it to an external disk, use [opt] button during boot up, select my external disk and install clean from it…..??? Yes???? I just want a clean install and I guess I'll wait for a new, stronger, faster, better, computer for Yosemite… Obsolescence…. How sweet it is. Some smart one give me a quick answer…. Thanks Loads. I ment to say 10.8 not 10.9….. My Bad!!!