[Zubersoft]

Without knowing more, it sounds almost like the Android 9 is processing the hashes for all of the files in the entire library. This can take an exceptionally long time if the device has slow I/O (which can happen with an SD card) if you have a 10 GB library. You may just need to wait a very long time for it to proceed, but this would be a one-time thing. If you backup one of the other devices that can sync properly and restore that on the Android 9 device, then the sync should go at the normal speed, as the hashes would be saved in the database.

Mike