Att använda en extern hårddisk kopplad till en AirPort Extreme för backup mha Time Machine är en bra idé. Vad som är mindre skoj är att den initiala backupen på kanske flera hundra GB tar en evighet att genomföra. Den första backupen görs därför med fördel via FireWire 800, men tyvärr är det inte så enkelt som att bara koppla om disken. För att det ska funka måste man göra på rätt sätt. Jag hittade en bra guide här:
Prep the Disk on the Network
You'll need a backup drive (preferably erased), Mac OS 10.5.2, and an Airport Extreme running firmware 7.3.1. (Later versions might work, too, in both cases.)
Hook your backup drive to your Mac, wait for it to show up in the Finder, then eject it. (This causes necessary hidden files to be written to the drive. You probably already have those files, but do this anyway, just in case.)
Hook your backup drive up to your Airport Extreme.
Navigate to the backup drive in the Finder. That makes it available to Time Machine.
Open Time Machine preferences.
Turn on the "Show Time Machine status in the menu bar" option. You'll use it later.
Select "Change Disk." Your backup drive should be listed, with the name of your Airport Extreme in parentheses. Choose it.
Select "Back Up Now" from the Time Machine menu bar icon.
You should get a progress bar that says "Preparing..." This will take a while--mine took about five minutes. Wait for it.
When the progress bar no longer says "Preparing..." and is showing data being copied, press the little X to cancel the backup. (Note: don't cancel while it says "Preparing!" This seems to get your backup into an unrecoverable state, and future backups will "Prepare" forever. You can fix that problem by deleting the backup image.)
Turn off Time Machine. This will prevent backups from triggering inappropriately during the rest of this process. (Note: to avoid the indefinite "Preparing" bug I just mentioned, don't turn off time machine until after you cancel the backup!)
In the finder, navigate to the root of your backup drive and open the Time Machine disk image. Mine was named Jim's Computer.sparsebundle.
Navigate to the newly-opened image. It will have a single directory in it named Backups.backupdb. Delete it. That's right--you'll now have an seemingly-empty disk image. (In reality, there are some important hidden files left behind that make this trick work.)
Empty your trash.
Do a Local Backup
Eject the backup drive in the Finder, unplug it from your Airport Extreme, and plug it into your Mac.
Select "Change Disk" in the Time Machine preferences and pick the backup drive again. This will automatically turn Time Machine on; turn it off again.
Select "Back Up Now" from the Time Machine menu icon.
Wait for the back up to complete. Rejoice as it takes hours, not days.
Move the Backup Drive to the Network
Eject the backup drive in the Finder, unplug it from your Mac, and plug it into your Airport Extreme.
Navigate to the backup drive in the Finder so it will show up in Time Machine.
Select "Change Disk" in the Time Machine preferences and pick the backup drive again. This will automatically turn Time Machine on; you can leave it on this time.
Select "Back Up Now" to prove that it works. It will spend a while "Preparing" (mine took about two minutes) and then you're done!