This has been an issue since day 1, rather than provide the workaround I provided, rekordbox seem to want to just let people struggle.
im sure they're working on it, but if you search through this thread you'll find a post of mine on how to fix it (a solution that works, but it doesn't fix the issue rb has)
failing that, google how to create a symlink on Mac of your hdd replacing the original music folder :)