Playback Error Conversion failed. The transcoder exited due to an error

[drmagoo]

As always, Permissions. PMS is not allowed to write into your transcoder temp directory.

Aug 20, 2018 21:45:46.579 [0x717ff400] ERROR - Error creating directory "/media/pi/gamera/transcode_temp/Transcode/Sessions/plex-transcode-17c34871-fafb-49e8-a166-7f4591ad1376": boost::filesystem::create_directory: Permission denied: "/media/pi/gamera/transcode_temp/Transcode/Sessions/plex-transcode-17c34871-fafb-49e8-a166-7f4591ad1376"

Just add write permissions for PMS user plex to /media/pi/gamera/transcode_temp.

Please also read this guide that will tell you how to correctly prepare your external drives for PMS:

Linux Tips

Using other hard drives (Windows or Linux) with PMS This How-To is written to help with Internal and External drives. It also is written to demonstrate how to cope with NTFS and EXT4 formatted drives. By logical extension, any format supported by Linux can be handled using the technique shown here. In all cases, the goal is to give user plex permission to read your media files which Linux doesn’t allow by default due to its strict security model. We will walk through the steps needed …