So Plex has worked for me for a long time on my raspberry pi 3 with raspbian. Recently my root was full so I decided to move all te app data, but something went wrong and I had to remove plex with all the metadata. I have my media on an external HDD. To prevent filling my root again I decided to store the metadata on the HDD too. But I could not get it to work. Now, I have been searching on the internet for a solution, but I was unable to find a solution for my problem. Playing the media works, but now there is no metadata, even after more than 1 clean install. I have found the /lib/systemd/system/plexmediaserver.service file and I noticed that the Application support directory is missing: /var/lib/plexmediaserver does not exist, even after a clean install. Could this be the problem? If so, what can I do to fix this? A clean install does not work… If you need more information let me know 
