I've been using my MBP for a few months now and have a 250gb external hard drive that I've used in the past for backup. Everything was fine and work perfectly but I'm confused what exactly is being backed up. On my MBP, have about 175 gb of files and music. It's mostly music which takes up about 145 gb of hard drive space. One thing I noticed when I was using Time Machine before is that it ran out of space very quickly. I believe it kept making copies of my files which I'd rather it didn't or however it works. How can I keep Time Machine from making numerous copies of my files? Does it not sync it with the laptop and only add the updated or added files? I'm unclear about how it works if someone can shed some light for me on how to use it properly.