There is a feature in Mac OS X 10.5 which gets mentioned a lot lately: Time Machine. Unfortunately, Apple has done a good job of grabbing everyone's attention with the user interface of this thing, but the key advance is not the looks but the inner workings of it all. What TM does, is maintain hourly/daily/weekly rolling backups. Which is great, except that until now you always had to somehow keep system load and disk usage in check. So normally this approach simply doesn't scale well enough to be used on an entire disk, no matter how clever you wrap up rsync, etc. Which means you end up with a diverse set of backup strategies. Messy, tedious, and brittle, alas.

Well, times have changed. TM is revolutionary, because its overhead is proportional to the amount of change. Every hour, my (quiet) external HD spins up, rattles for a few seconds, and then spins down again after 5 minutes (not 10, see "man pmset"). All you need is a disk with say 2..3x as much free space as the backed-up area. One detail to take care of is to not include big changing files such as VMware/Parallels disk images and large active databases in the backup, because TM backs up per file. So I put all those inside /Users/Shared and exclude that entire area.

Oh, and TM does the right thing: it suspends and resumes if its external disk is off-line for a while, such as with a laptop on the road.

Note that attached disks cannot protect from malicious software and major disasters such as a fire - but this is something an occasional swap with an off-site disk can take care of.

I no longer "do" backups. I plugged external disks into the two main Macs here and TM automatically asked for permission to use them. End of story.