Unable to transfer files on android 8.0 TV box to the USB
Have you tried TotalCommander?  Its possible that the USB drive is mounted as read-only.  In TotalCommander, If you copy to the USB drive and the USB drive is mounted as read-only: TotalCommander will automatically ask if you want to remount the drive as read-write, which is what you want.  Say 'Yes', then it copies whatever files you want to your USB.