I've looked for the answer to this, but haven't found it. How big a hard drive should I have for a time machine backup partition? I have a 500 gig FW800 external hard drive, and want to partition it so it's not completely used up by time machine. My MBP's internal HD is effectively 125 gigs (160 gigs total, with 32 gigs used for Boot Camp). To back up those 125 gigs, how much external HD space should I allocate? 125 gigs? 250 gigs? Something in between?