You've got to love this software! Any photographers out there using a Mac and doing manual back-ups of their images onto DVDs and external hard drives should take a look at Carbon Copy Cloner, from independent software developer Mike Bombich. Mike has been working for eight years to create a reliable back-up and cloning solution for the Mac. He offers the full version for free, in return asking for a small donation to enable him to make further development sustainable. You can create an exact clone of a hard drive, and then schedule regular (daily in my case) synchronisation, so that new files are backed up incrementally and deleted files are also removed from the clone drive. The interface is extremely intuitive. And once you've checked all your boxes and set it up like you want it, it even has a section down the bottom called "What is going to happen". I love it!
You still of course should have an off-site copy of your files too - I burn mine to DVD and store them at my mother-in-law's house - but having a bootable back-up of your digital life right to hand is the way to go.