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[tom80H]

You’ll need to give Plex permission to see/access those files.

Synology FAQ - Questions, Answers, and How-To's

FAQ 19: Setting permissions to allow Plex access to media files Before beginning, it’s important to remember one key point; Plex doesn’t have any special privileges on Synology except for being able to access the Video Transcoding acceleration hardware. It cannot read any of your files unless you expressly grant permission. This is a fundamental principle of Linux (which Synology uses for DSM) Because of how Synology grants permission, access to files is broken into two different …

IIRC, Synology DSM can be „special“ when it comes to permissions assigned to its default media locations.