I believe this is something new, but may have been going on and I didn't notice until now.

I am running 19 Data Drives that are pooled into a single drive with Stablebit DrivePool. I have Mezzmo pointed at the single media folder on the drive letter created by DrivePool, in this case v:

An example of the location of a typical file for a TV show would be v:\ServerFolders\Brettflix\Television\D\Dead Like Me\Season 1\Dead Like Me S01E03 Curious George.mkv

I have a holding folder v:\Waiting Room\Television where I temporarily place media so I can check to see if the .nfo and art is complete and correct before placing it in its permanent location. When I put the "Waiting Room", Mezzmo has no problem detecting the files, reading the .nfo file and putting it properly into the library's playlists.

As soon as I move it to the permanent storage folder (similar location to above), Mezzmo refuses to recognize it as a valid media file. No amount of scanning gets it into the library. It just sits there and looks like this:
Mezzmo TV problem.jpg
If I click one of the files for its <Properties> Mezzmo says it is an invalid media file.

If I copy and paste the folder back into the Waiting Room, Mezzmo immediately detects and populates everything
No Problem in the Waiting Room.jpg

This has to be some file system thing, right? My library is quite large, so am I running up against a limitation on the number of files that can be in any single directory on a Windows PC?

Right now I am splitting my <Television> folder into 2 different Windows folders and that solves the problem but could complicate finding media in the future.