This is also part of what Time Machine can do for free - boot from your OS X DVD and under Utilities, you can restore from Time Machine. So you'll lost (at most) 59 minutes and 59 seconds of data, as long as the drive gets plugged in and backs up.
Time Machine was introduced in Leopard; he's using Tiger.
That said, the best solution is almost certainly to upgrade to Leopard (or Snow Leopard, if you can wait that long) and then use Time Machine. It's almost completely automated; you basically plug in the drive, open the Time Machine preference pane and click 'go' and then it'll back up on an hourly basis to that drive whenever it's plugged in.
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