How to use a USB drive that already has music loaded (i.e. without exporting all the music)

No, sorry, rekordbox requires re-export much in the same way an iPod works with iTunes. We recommend using two separate devices; one for your library, one for export.

Ok. I thought so!

Thanks

So when you export to a device, the exported files dont show up as mp3s/wavs?

Yes, they are in the CONTENTS folder.

To add, rekordbox doesn't cross-convert your files. Whatever format they have in your collection is what type they will be upon export.

I've never understood why Rekordbox works like this. It adds such a big workload to the procedure, having to copy all your files from one drive to another. My files are already where I need them. I don't want to copy them to another location.

Why can't it just scan the files, analyze them and create a database? Surely that's quicker than also copying gigabytes of data?

Other manufacturers library s/w can do it, DJ software can do it....   Why is Rekordbox the odd one out?

Other software is just that -- software. rekordbox is a music management software relying on nothing but a USB device.