I just went from ML to Yosemite. I used a new SSD so that I can revert back to my old system when (if) I want. I also want to keep my old backups done with Time Machine. I use an external USB drive for the backup and would like to continue to use it. The OS install and my important stuff doesn't take much space right now, maybe 20G. Q. Is there a way to have Time Machine not delete the old ML backups and continue to use the same drive to backup Yosemite? I kinda doubt there is, so the 2nd question, is there a way to copy the backup file somewhere else? If I refuse to erase my ML backup, am I forced to use another device for backup? I might be willing to erase the ML backup after I'm sure everything will work fine, so maybe using a USB stick as a backup will work for a while.