The 7.4.1 firmware update for all Apple 802.11n AirPort base stations – any model released since 2007 – combined with Mac OS X 10.5.6 may fix Time Capsule disk corruption problems that some people have experienced. Colleagues Shawn King (Your Mac Life) and John Gruber (Daring Fireball) have recently explained to me problems they had with Time Machine disk image integrity and kernel panics. I also saw repeated disk image corruption in my testing last year.
While Apple provided no details on how or why corruption might occur in disk images that Time Machine writes to a Time Capsule internal or external drive, this update is designed to avoid problems that led to such corruption. If that sounds overly circumspect, well, we are talking about Apple here, but I was assured that a small number of edge cases like mine should now be resolved. (Note that Apple still officially supports only external drives connected to a Time Capsule for Time Machine backups, not drives connected to an AirPort Extreme Base Station.)
One piece of advice if you’ve had problems in the past: Back up any existing Time Machine disk images to an external disk using the Archive feature in Disk Utility, erase them from the drive, and start fresh with new Time Machine backups.
In some cases, mounting the Time Capsule volume via AFP, launching Disk Utility, dragging the corrupted disk image into the program, and then using the Disk First Aid > Repair Disk feature may correct errors, although it could take many hours for a large backup disk. I recommend the fresh start approach for most people, though.
Time Machine works as an incremental backup system, writing all files on a selected system to a disk image in a first pass, and then creating only copies of files that have changed each hour while Time Machine is active.
But Time Machine also creates what’s effectively a snapshot of a hard drive for each backup, which necessitates making vast numbers of hard links, which reference any unchanged files. This can be difficult to do reliably over a network and may have been part of the trouble with corruption occurring over time.