Last updated on December 23, 2008
I just acquired a new external disk enclosure and 750GB disk for hanging off of an AirPort Extreme and using for Time Machine backups of my main machine. From this I currently have ~480GB of data to back up, and for some reason the initial large backup repeatedly fails when I attempt to do it over the network.
The easy way around this is to first do the backup to the drive when it is connected locally and then hang it off of the AirPort Extreme to continue the incremental backups. The problem is that this doesn’t work as one would expect, because when an initial Time Machine backup is made to a local disk the backup ends up in a series of subdirectories, which is a different format from what it is via network.
When the backup is made to a volume hanging off of an AirPort Extreme a .sparsebundle file is created containing the backup; essentially a disk image stored on the network. Therefore, if you make a Time Machine backup locally and then try to use it via an AirPort Extreme the .sparsebundle file will be created on the disk in parallel to the now-useless directory structure.
So, how do you work around this? Easy. Hook the external disk up to the AirPort Extreme then either let the backup fail or cancel it, which will leave the incomplete .sparsebundle file on the disk. Disconnect the drive from the AirPort Extreme, connect it to your Mac, and point Time Machine to that volume. If it finds an appropriate .sparsebundle on the volume (which it will, since it’s already there) it’ll use that instead of creating the aforementioned subdirectory structure.
The backup will then happen quite quickly, and after it completes you can just hang the drive back off of the AirPort Extreme, redirect Time Machine to back up to that network volume, and things will continue via the network.
UPDATE: Since 10.5.5 was applied to my machine I have been unable to use this backup method and have had to resort to making the entire initial backup via network.