May this be the last question I have for the week! I have an older iMac that I want to upgrade to Yosemite. I have to clean off the hard drive too. I have a drive I will use for time machine back ups and anything else necessary. If I want to start fresh and upgrade to Yosemite... Is there a way to do this using the hard drive? If so, how should I partition the drive first? Can one download the OS to the back up drive? Or is there a better way to do this? Thanks!