This is the correct solution for unattended file copy/sync, plug the external drive and wait till the copy finishes. The only difficulty is monitoring progress/completion/failure if that’s on a headless machine. I built a basic orchestration system with systemd .path/.mount/.timer/.service units, logging and monitoring around this, once you understand your exact needs and get it working, it works.

The free edition of Mattermost is artificially crippled [1]. Amongst other things, only the most recent 10k messages will be displayed, older messages will be hidden (borderline ransomware). They took a bad turn when they decided to start LARPing as a US defense contractor company (I mean look at this homepage [2], wtf). I wouldn’t recommend it for a new deployment.