Just was reading this topic because I had the same problem and it fixed it for me, but I have a related question. I plan on transitioning all of my music off of a hard drive and on to an SSD soon. Does the SSD need to have the same drive letter for all my music/playlists to stay working? If so, that might be difficult because I plan to keep the hard drive connected for movies and tv files.
The only safe way to make the transition would be to edit the Plex database directly, and change all file paths to the other drive letter. But only for music items.
You will have to dive deep into the structure of the database to pull that off without side effects or data loss.
Make sure to always keep a backup copy of the database.
Shut down Plex server before making the changes. Copy the music files to the new drive before editing the database.