I have spent many hours searching the internet for information but still can’t find anyone who can help me.
I can’t add local external storage. As I understand it’s because I don’t have permissions to the file I want, but no matter what I do, nothing helps.
Sorry i do not use Docker and Portainer. But i think /hdd is an own partition. It could be that simply changing the user rights on the Nextcloud user is not enough. Maybe you need to include it differently e.g. in /etc/fstab or within Docker and Portainer. Search in this direction.
External memory limits you. Maybe it makes more sense to increase the primary memory if possible.