How do I back up my Mac to an external drive using time machine when I already have files on my external drive. I ask this because I am currently using a PC and manually backing up to my external hard drive. However, when I do make the switch to a Mac, I would like to still use the current hard drive I have. So is it possible that I can safety set up Time Machine with a Hard Drive that already has a lot of files on it? Or do I have to reset the drive and start fresh.