I believe that's an option you can set in the Android preferences for the app.
Rekordbox Android App v3 & SD card
Hi, is there an intention to allow import/storage of files onto an SD card in Android in a future release of the new v3 Android app.
You believe right so, that Android has this option. But it is by Rekordbox to decide to enable it, but as with all this company does, it is just missing necessary features. Without this, it just makes no sense in using it.
So the answer to this is not "I belive blabla", but rather Rekordbox just doesnt think about what they deliver and it isn't possible!
Simple functionality, like changing the storage, is just nothing that they are providing.
The usability of the app goes to zero without this, for basically an Android user.
I also would wish that if someone reaches out for help, that there would be at least a bit of effort to try helping your customers. An answer that starts with "I believe..." shows the effort that is put in helping them.
So another zero here.