I have been using Plex for years and never had a problem streaming files from an external hard drive attached to my Mac desktop. But now the drive will no longer show up as an available option. Any suggestions?
That sounds like a problem somewhere in your setup or configuration. I do not see any way Plex could cause this kind of issue.
But, if it is something with Plex, you will need to post details of your setup and your log files. Without those Plex can tell nothing about the problem.
Make sure Plex has permissions to removable drives in files & folders in System Preferences > Security & Privacy ( or give it full disk access)
thanks