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What software can I use to sync two folders that can't be seen at the same time, by transferring just the changes on a portable disk?

The scenario: I have a collection of music at home that I occasionally add to, and a collection at work that I occasionally add to. The two machines are never in direct communication, but I can carry a removable disk between the two, so the changes made on either could in theory be propagated.

An ideal solution would be one where I could do things like choose a subset to keep at work.

Clarification: The entire collection is too large to fit on the portable disk

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This does not cover the ideal case, but it allows you to sync both libraries:

The tool Unison can two-way(!) synchronize two directories. So you can run unison at home to copy all your music to the portable disk. Then go to your office and sync the disk with your collection there (copying your new office music to the disk and vice versa). At home you can again sync it there and get your new office music to your home PC.

Regarding your second question: Not sure what else unison can do, but I know you can permanently ignore directories during sync.

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The entire collection is too large to fit on the portable disk, so I'm looking for something that will just copy the "deltas". Sorry, should have made that clear! – pufferfish Oct 14 '10 at 9:00

You don't mention what you're using to manage your music collection. However, I'd create a playlist just for the office (e.g. "Work_Tunes"). That playlist could reside on the removable media, sync'ed into a separate folder by your favorite software (I use "iTunes Sync" with a folder for this).

What I'd do is collect everything added at work onto the disk, sync it up at home, they sync your playlist folder (after making any additions or removals) and carry it back to work. I'd keep the work computer clear of extra music if possible.

If each computer had internet access, Dropbox could take the place of the removable media. No need to have both home and work computers online.

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