Okay, the permission error is almost certainly because the Samba process inside the container doesn’t have the right Linux permissions for the host directory /mnt/my_ext_hdd/my_dir/my_subdir.

• On your server running docker, find the numeric UID and GID for that directory: ls -ln /mnt/my_ext_hdd/my_dir/my_subdir

• you likely need to set PUID=<uid_from_step_1> and PGID=<gid_from_step_1> in the environment: section of your docker-compose.yml file for the Samba service.

• Recreate the container (docker compose up -d --force-recreate).

WARNING: This assumes you are only accessing Samba from within your secure local network. Never expose Samba directly to the internet. Doing so is a major security risk and makes you a target for attacks.