DSM 6: Do not put your media in /plex. That location is reserved for Plex use. Doing so will cause problems.
DSM 7: /plex no longer used for Plex data. OK to put your media there.
When you install Plex for DSM 7 the migration process will move the Plex data from /volume1/plex to the new location.
See this thread for additional information:
