For Plex to be able to access media on external USB drives, you need to create a dedicated mount point. Use chown to give the 'plex' user ownership of the mount point. Add a line to the /etc/fstab ...

If you stream a lot of content, you'll want to use a Linux distribution with certain qualities. Here are my favorite distributions for streaming.

It's simple to turn your old laptop into a powerful Plex server, and you can use Windows or Linux to get the job done.