Official comment
Hey Michiel,
You could do exactly as you described, but don't just move the rekordbox library - once you're running from that database, also EXPORT the entire collection to that drive. Booting rekordbox without the drive connected will give you a blank database (and you can always toggle over to the external drive as the database if you want), but with the drive connected, you can play from that export device, the only thing that wouldn't be saved is the play history (songs aren't logged or turn green to indicate they've been played). But! The other thing is you can import the songs back into your collection and it will retain the beat grid and hot cues.