Mount bind NC/subfolder to sdcard/folder permission problem

On server (ubuntu 16.04) once I mount --bind /var/www/nextcloud/data/username/files/subfolder /mnt/sdcard/subfolder
I can no longer write to … /files/subfolder. permissions are 770 root:sdcard
chmod/chown wont work because the sdcard is fat32

I am desperately trying to expand storage available to Nextcloud.
obviously the next thing to try is to format the sdcard to ext2, but this would be a total pita, so I would like to know if there is something I am missing that won’t make this work. Thank you!