I have spent some time trying to isolate this problem.
I am running Plex Server on a Windows 2012 Server which is running on a very fast rack-mount machine.
When I scan movies that are on the local disk, or on a TeraServer NAS, only about a third of the movies are recognized. If I scan the same directory off a Seagate Home Cloud, all the movies are seen.
From what I can see, the Seagate Home Cloud takes much more time to return directory results and in general runs much slower than the TeraServer or the direct hard drive on the server. ( Both the TeraServer and the server hard drives are set up in RAID configurations)
When I run “Scan library files” off the Seagate, the scan is slow, but it finds all the movies.
When I run the same scan off the local hard drive or the TeraServer, it flies through the scan but misses movies.
I suspect this has something to do with caching of the directory results, and the timing of the scan. Has anyone else run into this problem, and does anyone have any suggestions? I would prefer to run the Plex library off one of the faster RAID drive systems.