I have mac mini 2011 8gb ram upgraded from 2gb and 500gb hd. Intel HD graphics 3000 512mb and 2.3ghz core i5. Anyhow my mini came with 10.7. And I upgraded to Yosemite now. I backed up the 10.7 to an external disk and did a fresh install of yosemite no the internal. Can I use the same hard drive with my time machine backups from 10.7. Is it ok to mix and match. Or should I just keep 10.7. on a separate hd and get a new hd for yosemite time machine.