You can only have a single Plex Media Server installed on a computer. The only option I can think of to run two Plex Media Server instances on a single computer would be to run some form of virtualization software to have a second VM running whatever OS to run the second Plex Media Server instance. Why do you want a separate server for Music? Why not combine everything into a single server? I have Music, Movies, and a couple different TV libraries all on the same server. Put your music on the secondary HD and create a Music library on your existing PMS pointing the Music library to a folder on the second HD.
