Thanks, yesternow, for the reply. I checked permissions on the Sandisk USB drive. The Movies and TV Shows folders both have drwxr-xr-x (755) permission. The media files all have -rw-r–r-- (644) permission. This matches the permission when I copied the files and folders to the /home/Rpi/Videos directory.
Please check the whole chain from / to the media files. Linux permissions are not propagated to /, i.e. they are not inherited. Good test: start a bash as user plex and try to access the media, e.g. with sudo -u plex /bin/bash
Btw: here is the perfect guide to avoid all problems when mounting external storage: