So far I've been unable to add any USB external storage (as internal, not removable) AND have Plex Server operate. If I configure Plex Server and then add an external USB drive as internal storage, it will crash and I have to remove the USB and move everything back to the internal 16GB card, then delete the cache/settings and go through the setup again. If I have the USB drive already added to internal storage and then try to configure Plex for the first time, it will crash during setup.
I only have one SSD external drive (with multiple caddies - so the caddy isn't the problem) to test with, but it works with all other computers I own. I'll continue troubleshooting, but any help would be appreciated!