I noticed this thread since I just bought an Android device (second hand & rooted) to test the Rekordbox workflow some more. Terrible app, yet I can't complain. IT'S Free!!! :-D
Seems pioneer is still in saving mode. I guess the C.E.O.'s have to skip a holiday this year and the App department had to stop developing for a while, to save some money and make sure our favorite hardware vendors will not go broke. Anyway, some updates, but we didn't buy enough gear to really upgrade the software to the specs of 2016 (Instead they go back about four years. As does this thread.)
But hey! Android is like Windows XP. We like that! (not). Anyway, it means we can 'mod' away. And just for your info: I found the right mod (took me two weeks and I didn't check if it was mentioned here already).
I started trying to work with a program that creates the symlinks knowing that this must be the technique used to pass files through to the Ext. SD. But I wasn't happy with it. It also created all kinds of double entries in diverse Media Databases. One of them being MediaMonkey, which I really need as the core of my Media management. Managing your library can better not be done through Rekordbox since it doesn't have any tools to service the back end (database) of that library. So there's only one way if it get's screwed up: throw it away and start again.
I only use it to read, import/ export Playlists from an iTunes.xml and I am a very happy user of the Sync Manager. Unfortunately not used to read from your Android or iOS collection database so it cannot keep playlists in sync for the latter.
Now, here is what you can do to give any Android App in a UAC version of Android (Kittkat and above I think I understand). You only need to add one line in a file called:
'Packages.xml'. The folder hides in a rooted device in:
Copy the file to a place where it is easy editable with f.e. notepad++. Rename the file to PackagesNEW.xml and then do the folowing:
1. Search for pioneer in the file, it will find the name of the .apk package installed.
2. There you should see some lines between <perms> and </perms>. That is where the permissions will be set on boot.
3. Add: <item name="android.permission.WRITE_MEDIA_STORAGE" granted="true" flags="0" /> to the rest of the permissions.
4. Save the file and move it or copy it back to the original location in the root of your device, /data/system/
5. Rename the existing file to PackagesOLD.xml (and save it for backup if you wanna). Rename the new file to Packages.xml.
6. Reboot your device
7. In Rekordbox App settings point to the new library location; /Storage/ExtSDCardName/(anywhere).
This can be done for any application on a rooted device. Rekordbox will now have read and write(/changes) access to your beloved External SD.