Official comment
You can copy the music from the export drive back to your internal drive (or another drive) and then bring that into rekordbox. You can also load your export drive in rekordbox to view the playlists, but there is no way to recover the beatgrids, hot cues, memory points, or playlists automatically - you will have to rebuild those manually.
And yes, a backup is critical. My preference is Backblaze; it works in realtime to the cloud, unlimited storage for about $4/mo.