I’m ready to purchase a NAS drive with Plex Media Server (PMS) already installed on it. Will PMS be able to scan all my existing NAS drives i.e. all the NAS drives connected to the router and those NAS that are connected to mains “powerline” devices ?
I’d say it will depend on if you can set up a way for the new Plex NAS to “see” the files on those other NAS devices.
I don’t see any way it would or could “automagically” find and scan them. How do you see the files on these other NAS devices from your computer today?
How you go about mounting those other NAS drives on the Plex NAS so that Plex can scan them will depend on the specific brand of the Plex NAS. I googled mount one nas drive on another and saw some links for synology and qnap NAS devices.
Once mounted you will need to add the paths to the folders you want scanned to the Plex Libraries you create on the Plex NAS. Folders with movies will have to be added to the Movie libraries and folders with TV shows will have to be added to the TV Show libraries, etc.