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I have a folder on Drive 1 called Music, and it is 100GB. I have a folder on Drive 2 called Music, and it is 116GB. It's the most updated version. How do I copy the Music folder from Drive 2 to Drive 1 without having to rewrite every single file?

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Which version of OS X are you using? – Nix May 13 '12 at 21:59
I am using 10.7.4. When I copy the folder from one drive to the other, I'm asked if I want to replace. No merge option. – dallen May 13 '12 at 22:02
up vote 3 down vote accepted

The merge option will only show up if you are copying (rather than moving) the folder. Make sure you hold option while you drag the new folder to the location of the old folder. Alternatively, using +c to copy the new folder and +v to paste over the old folder should give you the option to merge the files.

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This still seems to write over files that don't need to be rewritten. Using rsync instead – BRogers Mar 9 at 2:44

This is a classic use case for rsync:

rsync -av /source-path/source-dir /destination-path

rsync will copy only new and changed files to the new location.

It is important to understand how a trailing slash on the source argument functions. If there is a trailing slash then the contents of /source-path/source-dir will be copied to destination-path. If there is no trailing slash then source-dir itself will be copied to the destination and its contents will be another level down in the destination hierarchy.

So if you want to replicate one path to another include the trailing slash as follows:

rsync -av /sourcepath/sourcedir/ /duplicatpath/sourcedir/
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Great catch! And if you're doing this from multiple drives (or anywhere on your computer) and don't want to type out the directories you can just drag a drive (external usb, or time capsule) or just a specific directory and have it add it to the command for you. Just drag the target location into terminal – BRogers Mar 9 at 2:46

Not sure why the default Lion Merge button doesn't show up for me. But I ended up using ditto. Found here:

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BE CAREFUL! ditto does not keep newer files, it always overwrites destination with source. If source file is older, it will zap away the new file and replace with older file – FrancescoMM May 18 at 15:55

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