Situation…

My FreeNAS USB stick died and I had no backup of the FreeNAS configuration file (school boy error).

I’ve imported my ZFS pool and imported my jails (can’t remember how – It was a long time ago) but I’m unable to update my jails software because they weren’t installed via plugins.

Solution…

Install the plugin correctly, copy the plex data from the old jail to the new jail (from Outside the jails!)

# cp -R /mnt/zfs1/jails/plexmediaserver_2/var/db/plexdata/Plex\ Media\ Server/ /mnt/zfs1/jails/plexmediaserver_1/var/db/plexdata/Plex\ Media\ Server
From inside the new jail
# cd /var/db/plexdata/
# chown -R plex:plex  Plex\ Media\ Server

plexmediaserver_2 is the imported jail

plexmediaserver_1 is the new jail

Unsure why the numbers are counter to what you’d expect.

References

https://support.plex.tv/hc/en-us/articles/201370363-Move-an-Install-to-Another-System