I’m using OSX, fully updated, and my Plex is the latest version.
I have an external drive connected via USB. OSX Finder can see it (and the files on it) just fine … but when I go to Manage Libraries/add library - it doesn’t show up as an option.
I do have Plex authorized to read/write external drives in OSX Privacy and Security.
Please share your actual macOS and Plex Media Server (PMS) versions.
Can PMS interact with your Macintosh HD partition?
Also, what filesystem type is the disk partition(s) formatted with? FAT32, ExFAT, APFS, HFS+, etc.? That’s more a curiosity, as if Finder is able to browse the disk and interact with the files then PMS should be able to as well.
Please share a screenshot of this so we can verify it is set correctly. If you changed this setting while PMS was running, try stopping and restarting PMS. It shouldn’t be necessary, but it can’t hurt.