You’re running into that permission error because of how Docker handles file permissions between the host and the container. It’s by design for security reasons. The user inside the container likely doesn’t have access to the mounted directory unless the UID and GID match what’s on the host. You can work around it, but it’s locked down intentionally.
Also, what’s the use case here? What do you need file sharing via Samba in a Docker container for? If it’s just about moving files in and out, docker cp or docker exec -it container /bin/bash might be easier.
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.