I was having trouble with my iMac and ultimately wound up doing a clean install of the OS. When I set up the new OS I changed the "home" name of the computer (Apple had set up the computer initially and used my email address for my Apple ID, which I found a bit annoying ). My backup drive only has about 6GB free, the rest is taken up with old backups. I would like to set Time Machine again to do automatic backups, but I can't figure out how to get it to use my drive and just erase the earliest backups leaving my most recent ones just in case I need them. Is there a way to do this or do I have to erase the drive and start over with a completely new backup of the new OS install?