Backing up external hard drive shared by two macs to network share without duplication In my household, there is a MacBook Pro and MacBook Air, both of which backup to a NAS via Time Machine. There is also a portable external hard drive that we use for storing large media files. I would like the external hard drive to backup to the NAS when it's connected to either laptop, without duplicating files on the backup. Is this possible?

The only solution I can think of would be to have one machine exclude half the contents of the external drive from backups and have the other machine exclude the other half, so each machine's backup sparsebundle would include half the contents of the external drive, but that's a terribly inelegant solution.
Another solution is don't use time machine for this type of backup.

I use Chronosync (a GUI for sync) and in this case the scheduler would just skip the sync if the volume is not available. If it is available it would sync the drive to the NAS. You could also use Carbon Copy Cloner for this function.

If you feel comfortable with the terminal and you don't want to pay the money for Chronosync or its ilk, then rsync and chron could also be used.

Good luck!