NzbDrone is now Sonarr - The Application Formerly Known As NzbDrone - Sonarr
Depending on how your external drive is configured it may be case sensitive in which case, /volumes/hd2/tv shows/Downton Abbey is not the same as: /Volumes/HD2/TV Shows/Downton Abbey The very first thing you should do is correct this. The other thing is fixing your Season folder format, change it to Season {season:00}