Watched State: Enable Sync Watch State & Ratings
Plex Documentation → Account Information → Sync Watch State & Ratings
It is an account, not server, setting. Have anyone you share with enable it as well. Not supported for Managed Accounts.
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If you have custom artwork, descriptions, collections, etc., you need to save the Plex Data Folder from the current installation. This also saves watched status and ratings, but it is still a good idea to enable the Sync Watch Status feature mentioned above as a backup.
Use Hyper Backup to make a backup of the current Plex Data Folder. You’ll then restore after re-installing Plex (that’s the high level, there are other steps). Do not just copy the Plex Data Folder to a backup location. Using Hyper Backup preserves file permissions.
See Move an Install to Another System. Be sure to disable Empty Trash Automatically.
The Synology FAQ - Questions, Answers, and How-To’s also has useful information.
