Look, under DSM 6 the file structure was the following…
So 1 shared folder (\Plex) and server software & data (media) were in the same shared folder.
Since the upgrade/migration, the file structure is the following…
There’s now 2 shared folders. The \Plex\ shared folder is still there but a new \PlexMediaServer\ shared folder has been created with the new version of the Plex server inside and all the data (media files) has been left in the \Plex\ shared folder.
At this point my comments were just:
1/ My Plex is now running fine but the data has not been moved from the old Plex shared folder. Is it normal ? I thought they were supposed to be transferred and we could even delete the old \Plex\ folder?
I’m just surprised that all the data stayed in the \Plex\ folder. It never migrated/moved to \PlexMediaServer\ . Only the \Library\ folder has been removed from \Plex\ .
So after reading “DSM 7 - Packaging 2.0 (PMS 1.24.2.4973) - Release information” I understand I probable misunderstood something in the information topic regarding the migration as “no”, media files must not be transferred into the new \PlexMediaServer\ shared folder. So it’s normal that the migration process left them where they were under DSM 6.
And this answers my previous remarks.
Thanks! 
