If that happens, quit i Tunes, connect your external hard drive to your computer, and reopen i Tunes.
Note: If you are making a copy to transfer to a new computer, you don't need to perform step 5 unless you want to remove your i Tunes library from the old computer.
Within the library, click on the circular arrow icon in the upper right corner. Manual updating is great if you need to force the update right that second, but for ease of use and really frictionless user experience, you really want to enable automatic updating.
This will trigger a manual update for that library, and Plex Media Server will rescan the directories assigned to that library. Not only is automatic updating supremely convenient for you, but it’s practically a necessity if you have other people in your house.
You can also transfer your entire i Tunes library from one computer to another if you are migrating to a new Mac.
Before making major changes to your computer, it's always a good idea to back up your data: The best way to back up your Mac is with a Time Machine routine in place, but you can choose an alternate backup option that's right for you.
To manually update your Plex library, log into the web control panel for your Plex Media Server.