I’ve always been a huge fan of Apple’s Time Machine, and until recently it hasn’t caused me any headaches whatsoever. I’m running 10.6.2 on my laptop, and my Time Machine backup is located on a usb drive attached to an Airport Extreme base station, and normally everything operates as expected. I was away for a while, and had gathered a fair amount of data I wanted added (automatically) into my next incremental time machine backup. Being the impatient person that I am, I decided to disconnect the backup drive from the base station and plug it directly into my laptop in order to perform the backup (faster, right?). In the TM pref pane, I selected the usb drive, and the backup worked as expected. However, when I tried to enter time machine, I could only see the current state of my computer — as if all of my backups had disappeared.
Needless to say, I panicked. What I discovered, after much googling and poking around, is the following:
- When you initially create a TM backup on an Airport-shared disk, time machine generates a .sparsebundle file which contains the backup directory inside of it
- When you initially create a TM backup on a disk directly connected to your computer (I haven’t tested this with firewire drives but I assume the same applies), time machine skips creating the .sparsebundle file, and just creates the backup directory at the root of the drive
Generally speaking, who cares? Well, if you do something like I did — that is, move from an Airport disk to a direct connection — you’ll notice some strange behavior: Time Machine will back up normally, but it will not see those backups when you enter Time Machine. It appears that TM will recognize the .sparsebundle file as being your backup file when connected over usb to perform a backup, but it will only recognize the non-sparsebundle folder-only configuration when looking to browse backups.
Here’s the crazy bit — as an experiment, on my backup drive I mounted the .sparsebundle, and copied the Backups.backupdb to the root of the drive, then tried to browse my TM backups. It worked.
So where does that leave me? Well, I had always planned on moving my TM drive back to the airport, so I kept the .sparsebundle structure. I learned a new trick, which is option-clicking the time machine icon in the menu bar, and choosing “Browse Other Time Machine Disks”. That way, if you have the same setup I do, and you choose to directly connect the TM drive over usb, you can mount the sparsebundle and choose that drive to browse.
I still can’t locate information on why Apple decided to make TM operate differently over the Airport vs a usb drive. There’s likely a reason I’m not seeing which is why I’m hesitant to call this a bug or flaw. However something doesn’t feel right when you can successfully back up to, but not browse, a particular TM drive. Oh well.
Here are some links to stuff I found useful when performing my investigation:
- http://discussions.apple.com/thread.jspa?threadID=2057525 - Great troubleshooting guide for Time Machine, straight from the Apple Support forums. Covers just about everything.
- http://www.chrissearle.org/blog/technical/time_machine_large_external_disk_sparseimage – Seems to acknowledge a similar issue with only seeing one backup
- http://www.apple.com/downloads/dashboard/status/timemachinebuddy.html — Time Machine Buddy widget. Great to see what’s going on during a backup.
- http://www.soma-zone.com/BackupLoupe/ — BackupLoupe. You can see what was actually backed up in each of your backups. I used this to confirm that incremental backups were functioning as they should after changing my drive connections.
Cheers!