On DSM 7,
The username , which is a System Internal User (not a Local User), is PlexMediaServer
The errors writing in /volume1/Media require editing the Shared Folder permissions for Media.
In this case,
Use this FAQ to CREATE a permission rule for PlexMediaServer to Read/Write just as was previously done for “Plex” on DSM 6
This FAQ shows “Plex” (DSM 6). On DSM 7, we now use “PlexMediaServer”.
You will see “PlexMediaServer” in the dropdown list of available names.
To avoid confusion in the future , it is completely safe (and recommended) to delete user “Plex” in Control Panel - Users.
