I found this information that seems to be helpful (can't vouch for it, because I'm only now planning on doing exactly what you are -- albeit with a time capsule for backup instead of a directly attached drive).
It looks like Time Machine will exclude external disks by default. If you remove the external drive from the exclusion list, you should be all set:
- Open Time Machine Preferences
- Select Options...
- If the external disk is listed under "Exclude these items from backups" then remove it from that list
BTW, how is the performance if iPhoto with the airdisk? I tried it once and it wasn't usable, but that may have been an issue with the portable hard drive I was using. I've been holding off on getting a new time capsule in the hope that it will have USB 3.0 and eliminate the possibility of the USB being the performance bottleneck. Anyway, do you find performance acceptable with your library living on the airdisk?
DelegatorNH, thanks for the link, however, that covers adding an externally connected HD to the backup, not a mounted drive from a server. It seems TM gives you the option to remove a volume from the backup options, but it will not allow you ADD a volume, so it's pretty limited.
As far as performance, goes, I don't think it's too bad, although the wife is the one who uses iPhoto most of the time. It definitely takes longer to load the library on startup and save at shutdown, but performance in between seems to be acceptable. USB is not going to be the bottleneck, the network is. USB2 is capable of handling the task, even with a wired Gigabit connection.
My problem is the firmware on the drive being used as an AirDisk. It goes into sleep mode within 5 minutes of inactivity, then I have to browse through the Finder to wake it up so that iPhoto will see it. I think I'll eventually just setup a Mac mini as a iTunes/iPhoto server and that way TM will work and I won't have the sleep issues.
Salda, unfortunately my further research reveals no solution. The FAQ from the earlier link states that Time Capsule will not backup any network drive at all, INCLUDING the USB drive attached to the time capsule itself. This is a major bummer for me (as I'm sure it is for you as well). This is really a half-baked solution in my opinion. It means the USB drive is really only useful as extra backup space.