Official comment
An export drive can also be the source location of your collection's files and database, but I don't recommend it as it would double the used space (the original files + the exported files; no, it can't simply use the same ones).
Your export drive is not available in performance mode because the export database type is different - it's designed to be read by players. This is something we hope to see changed in a future update of rekordbox.
A second database is not available to be loaded simultaneously as the software is currently only capable of reading one database at a time. This loops back to the item above and is the main reason the export database can't be loaded. Again, we hope that this will be changed in a future update of rekordbox, as this functionality would allow another DJ to load their content for a back-to-back or DJ transition.
If you want to use your database and music on an external drive, as @Phillip says - it makes moving between computers convenient, you can follow the instructions in this knowledge base article.
