I get an error while trying to create an optimized version "optimized version could not be written to disk" Any ideas? I use a synology NAS
Plex can't write to any external shares. The only way to create an optimized version would be to use internal storage for all of your media so that Plex can write to the storage device. This is not a Plex problem, it's a limitation imposed by the Android OS.
Just FYI for anyone who comes across this post this is no longer the case. You you can write to your NAS now with your Nvidia Shield. This was released an update or two ago. The exact version number in which this update was pushed is in the support / manual webpage.
I was waiting for this update for a while so that I can manage my library (delete items) from my Shield.