So, I bought my wife a new iPhone. Before we attached it to her computer, I decided to update all the software, and (of course) the machine choked and died. I tried everything I could think of, but I couldn't get it back up and running. We took it to the Genius Bar, and the genius in question tried everything he could think of. No luck. According to our tests, the hard drive was fine, but Leopard would not install properly.
I decided to re-partition and reformat the hard drive. But, before doing that, I wanted to make sure the user drive was backed up. I took the 500 G disk we were using for Time Machine backups and attached it directly to my wife's machine, and copied over her user directory. Then I partitioned, reformatted the disk and reloaded Leopard. Everything went well.
My lovely wife walked by, accidentally snagged her hand on the USB cable for the hard drive, and knocked it to the floor. Now it only fell a few feet onto soft carpet, but when I plugged it back in, it refused to mount and only made a strange clicking sound.
All our backups were on that drive. The time machine backups. The new backups I'd just made. Everything. All of it gone. Poof.
Now, I have secondary backups for my most important files. But, alas, my wife did not.
It's easy to feel over-confident when you have backups. But backups can fail. Whenever you only have a single copy of your data, even if it's only for a split second, you're at risk.
Lesson learned. It's not a new lesson, but one I seem to need to be reminded of from time to time. Single points of failure are bad.