That is usually a permissions problem. Do you just plug it in and let Lubuntu mount the drive? If you do then the permissions will be set to your user to have access but not the plex user.
Linux has a very strict security model. No one user can see another users’s files without expressly granted permissions.
Linux is not a ‘Learn in a day’ OS because it was originally created in the command line-only days and has evolved into a server-rooted OS (enterprise grade)
If you are familiar with the Windows command line, I can offer three pointers
There are no discrete device letters. Everything is accessed through the file system itself somewhere. Every path begins with /
Windows uses \, Linux uses /
Both Windows and Linux are sensitive to spaces depending on context so surrounding path & file names with spaces embedded in them should almost always be surrounded with quotes "