From the book 'Mac OS X Leopard The Missing Manual', it says... Back-ups do take very little time, BUT, every hour, it's backing up EVERYTHING! My first back-up was 270 Gigs, which is what it was supposed to be, since it was supposed to back-up everything. The harddrive is 1 terabyte and with out me even being on the computer, each hour it added about 270 Gigs of used space backing up everything, even though nothing was changed. How do you get it to only back-up what's changed during the last hour? After my first six back-ups (2-4th ones deleted by Time Machine for lack of space), I finally added my media hard drive to the do not back-up list, knocking it down to 53 Gigs per back-up, which is everything on three hardrives, even though during an hour, not even close to 1% of the files are changed!