@wolf@mastodon.social All processes run "as a user" on your OS, and that user is used to determine the permissions that process has when interacting with the file system. You'll be running the server as if it had been started by the mastodon user, and so it will only be able to read/write files in directories that the mastodon user has been given permission to wrtie in. The server probably has a way whem starting it to specify which user it is supposed to run as.