I just realized that I actually don’t have Plex configured to use the USB storage. And Plex is using over half of the internal storage, so that might be why it’s not updating it.
PMS is small and should be installed on local storage.
I’ve seen this before, it sounds like your local drive is full and there’s no room for the update to download / no swap space left. Is your local storage full? Are you by chance using your local storage to store media files?
If your media is on the NAS and you have space available on the Shield, then your Plex Media Server on the Shield should be able to update… unless you’re also running Plex Media Server on your NAS and that version cannot update.
Have you tried diagnosing this from your Plex web interface? Log into your Plex dashboard from the web and see if you can manage your PMS from here. You can also see your logs from your Plex User Control Panel, the logs might be able to tell if you if you’re running into destination drive errors during the upgrade process.