How do I fix iTunes could not copy unknown error (-5000)?
The solution was simple - it was a permissions problem.  On the Mac, here are the steps I took to fix:


Note the song that fails, on my case it was a Zeppelin song.
Using the terminal, cd to the directory containing the song
Run ls -la and note the owner/permissions of the file that fails, in my case it was an alternate username I was using at one point, let's say that username is paulhewson, and your real username is hewson
Run sudo find . -user paulhewson from the top level music directory (~/Music), this shows all the music that is owned by this alternate user
This command will change the owner to your normal username: sudo find . -user paulhewson -exec chown hewson {} +


I suspect this error could result from any sort of permission problem, in some cases maybe read-only or locked files, in which case you'd want to chmod or unlock the files.  If there are multiple types of permission problems, you may need to repeat this process a few times.  Might be easier to chmod/chown everything in music, but I tried to avoid that so I didn't cause other issues.

Credit to this question for help with the find command to change ownership:
Change owner for all files owned by x