Hello, after much much hesitation, I think I can let go of Mountain Lion on my iMac late 2012, 3,2 i5, 1T FD, 8 Ram. I did this: I bought a 250 g external SSD and installed Yosemite 10.4.4 on it. I can now booth my iMac with the SSD. I am very pleased and happy with what I see and what I feel in Yosemite. A little faster than ML and no beachball from hell until now. I'm a bit restrained with what I can do since I can't install many apps on a small 250 g, but I can manage with Safari, Mail and Pages for now. Can't stand Photos though so I reverted to iPhoto. My question: it's been a clean install on the external SSD, what kind of installation would it be if I uppgraded from Mountain Lion 10.8.5 to Yosemite 10.10.4 from the App Store? A clean install? I read so many negative posts on Yosemite, I wonder if the kind of installation has some importance here? I have complete backups with Time Machine of my system. Lastly, will I lose for good ML? The ML installer on the invisible partition too? For whatever reason, could I use Time Machine to go back to ML? Thanks mucho for help and support.