Clean install/upgrade feelings aside, How does Time Machine work. When I plugged in my external HD, it automatically opened, but told me I'd need to delete everything off my external HD. What if I don't want to? I obviously have important files on there that aren't on my computer's HD that I just can't delete. Time Machine seems very confusing to me and I'm honestly scared to use it (for the reason above). What steps do I need to take? How does it basically work? And what kind of time investment am I looking at? BTW, my external has 180gb available (out of 250) and I have 50gbs on my computer HD. Thanks for the help!