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I'm not sure if this is a duplicate, but I'm having a really hard time finding a precooked solution. I have a desktop with 1TB hard drive and 3 laptops with 256GB each. I download a lot of music to all the computers at various times, so sometimes there are albums on one computer and not on the other, etc. I could store all my music (about 500GB) on the desktop, but it wouldn't fit on any of the laptops and I don't want even a fraction of it on any of them anyways. I want a folder on an external hard dive that can be checked against all the computers periodically to backup anything that's missing. So I'm not interested in sync'ing the folder with any of the laptops. Will SuperDuper, Carbon Copy or Time Machine do this? anyone using Git or any vcs for something like this?

I have the beginnings of a simple program to do this that is basically just diff -r x y | grep "pattern to match" | cp -r folder a location b but diffing that music folder with the one on the external drive is extremely slow. Also, I thought there might be a better solution out there.

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I always use rsync to achieve this. Assume that you mount your external HDD at /mnt/mydisk on all your laptops and all your music backups reside in /mnt/mydisk/music directory. And if all your laptop downloaded music files reside in ~/Downloads/mymusic, then the following command will do

rsync -az ~/Downloads/mymusic/ /mnt/mydisk/music

This command will sync the ones that are not available in /mnt/mydisk/music and available at ~/Downloads/mymusic/ You can check man rsync for more info.

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