It’s about file permissions, yeah, but the thing that eluded me for a few hours until I hit on this post, was that it’s also about mount points and security rules.
You gotta get the UUID of your ”plex files” partition(s) and edit
/etc/fstab to have your system mount it/them somewhere other than the default.
I think the example the Plex employee uses in his walkthrough is unfortunate because on my Pop_OS drive
/media is already used by the system. I created a new folder at the root level called
/plexmedia, chmodded it to 755, and listed it as the mount point for a partition on my external USB drive. When the system rebooted, suddenly the Plex server could see all my subfolders. Hooray.
NOTE: Before figuring this out, I had already tried the following; now I don’t know if they were unnecessary fiddling or if they were also a needed part of the solution:
(1) The drive and media files were originally part of a Mac setup. The drive was formatted HFS+, journalled. I took the drive back to a Mac and turned off journalling using
(2) Once I’d done (1) I could mount the drive in Linux in a write-able fashion and chowned the entire thing to user plex and group root. (As a read-only volume they had been showing up as ”503: root” where 503 is the ID of my everyday Mac user. I read some tricky posts about creating a Linux user with a matching ID and ”stealing” those ”orphan” files, but I’m glad I didn’t go down that route because the security/default-mount thing would still have been in the way… [I do think this mattered because once I discovered the Plex post linked above, I didn’t need to do the 755/644 stuff; it just worked.]