Be careful since a good backup plan is to have data on some other media type. An external hard disk is nice for quick day to day copies but things like power surge, virus, etc can wipe it out as quick as your main drive. Make copies on dvd, tape, flash, whatever and store offsite. I couldn't tell you how many times I've seen people with a bunch of external drives hanging off their system mess up and corrupt them all. Its espically important is your running a closed source OS since you never know what can go wrong with it.