im not sure, but maybe when you specify path, docker thinks theres two paths separated by :, also in linux to specify path you use forward slash /, so maybe puting in quote maarks would work, e.g. "D:\Pictures":/pictures/ and then looking for /pictures/ inside the container

You shohld also try checking logs when adding external library on immich with docker logs --follow immich_server

I started selfhosting month ago. I’ve came across Borg Web Ui. Its borg, but wrapped in nice web ui. I managed to setup daily local backups to external drive without struggling in the CLI. Borg web ui provides more dumbed down documentation (which is good for me). I’ve also setup to send notification to my phone when the backup is successful. I used NTFY for that