Symlinks Across Various Backup Disks/Volumes – How to Set Up

Another possible option: use rsync and a launchd job to execute it when the disk is mounted. I believe Time Machine uses a similar approach to running on disk mount. For the hourly execution, you can use cron. The discreet notification can be done in a number of ways (for instance sending a mail or the Growl extension which allows notifications to be sent from the command line). killall rsync should stop the backup safely if you need to pack up and go. Also rsync is also entirely capable of networked syncing.

It will take a bit of work to set up, but the advantages are:

• it's versatile (see man rsync, they pretty much thought of everything)
• it's discreet
• it doesn't cost anything
• rsync is quick

You may find this helpful for launchd jobs involving disk mounting.

Edit: I should add that the --partial option to rsync will mean that the backup is resumed after killall rsync.

Edit 2: You can use a Control Plane context instead of launchd if you prefer.

Yes - assuming you have OS X server running on the Mac Pro, you could share each drive under a different name and enable both for Time Machine backups.

You could then configure some portion of the machines to hit share A and the rest to hit share B. Since each computer stores backup contents in a sparse disk bundle, you could even shift a backup or two to the other drive and then reconfigure things if your initial allocation wasn't working after some time passes.