Synchronise/Merge MP3 collections without duplication or huge copies I have one drive containing about 90GB of MP3s which is used at home, it serves three Squeezebox players though the Slimserver software running on one of the desktop Macs.

I have a separate firewire drive containing about the same amount of music which I am very slowly cleaning up with Song Genie and I've finished the process of getting all the covers in better than 300x300 using CoverScout. Good tools if you are prepared to work more or less manually, they can come up with some pretty strange suggestions for covers &amp; tag info at times. All this is from CDs I have in storage and I've even scanned some of the covers I couldn't find online. It's a long-term project and one day I want to know all of it is perfect.

Some of the music on the home collection isn't on my firewire drive, and vice versa. If I copy all of the firewire contents over the home collection that would probably work. However I'd really like to know what music is in the home collection that I haven't got because there's no way I'm copying un-tagged un-covered music over my fixed versions. 

I could copy all the firewire drive MP3s over the home drive, then call the home drive contents the new cleanest and completest version but copy 90GB twice seems like a waste of time and there must be a clever way to do it?
Sounds like you need a merge/sync solution.  I personally often use arRsync - but there are lots.  Find one that allows you to prioritize one folder over the other when a match is found.  You may need to tag and apply album art to both copies, before they will be recognized as the same, though.