I thought I was pretty savvy re the iMac and I am very happy with my i7 16GB RAM and with 10.68. But all these "upgrades" have created a problem for me: 1. Some vital software will not run on 10.68 and requires 10.7 or beyond. I did install 10.7, 10.8, 10.9 and recently 10.10 on my MacBook Air (MBA) but did not feel it ↑ my enjoyment or efficiency. But my working computer is an iMAC. I did install Yosemite on a newly partitioned internal 2TB drive that came with the iMAC. 400GB in partition 1 for data, 600GB for newly clean install of 10.10 and 1TB remains unused but thinking maybe for Time Machine backups. I installed 10.10 just to see how it would perform on my iMAC as a bootable drive while preserving my 10.68 with all working apps. I did not install and bring over all my apps, etc from 10.68 to the 10.10 install. Now I see a lot of issues unresolved with 10.10 and want your opinion on: 1. Can I erase or overwrite the 10.10 drive with Mt. Lion or Mavericks? 2. What can you suggest on whatever I install to equate with a clean install? 3. Any other thoughts or recommendations?