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.

2 Answers 2


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!


Since we're talking about Time Machine, if it was me I'd only have the external drive backed up when it's connected to the Mac it's usually connected to. I know this isn't really what you're asking, but I believe it to be your safest option (mainly because it's simpler and you've got less room for human error). And, from my experience, external hard drives usually find themselves connected to one machine more than others.

However, if you find that the external drive is regularly connected to either Mac for prolonged periods and you're really needing it to be backed up regardless of the Mac it's connected to, then your terribly inelegant solution is really the only way to go. If so, you may want to rethink your file structure so that it's easy to "split" the contents being backup up by each respective Mac. (Assuming, of course, it's not already easy to split things up).

For example, you could split your media files into two main folders (one for backing up via the MacBook Pro and the other by the MacBook Air). Of course, these folders may have any number of subfolders.

Another example could be that all files are grouped into folders labelled as 0-9, A-M and N-Z (or whatever works for you).

Regardless, the idea is to make it easy for you to split them up within your Time Machine settings so that it's very clear which one(s) are backed up by one Mac, and which ones are backed up by the other Mac. Then you just keep organising your files within these folders as you need.

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