I have a LaCie external drive which holds all of my iTunes content (including music, films, tv shows and the library file etc). I'm going to buy another drive to backup the primary drive (so that I don't have a 'single point of failure'). After initially copying everything from my primary to my secondary drive, I want to keep the secondary drive in sync by just adding new or altered content to it once a week (ie. a differential backup). What's the best way to do this?