I recently got rid of my small (as in MB size) drives in my pool and replaced them with two 2TB 2.5" drives. These are plenty of space to hold all my backups and a DB duplicated music library. My music library is comprised of a root folder of "MP3 Music" with subfolders for each artist and album folders for each album under the albums artist. I use Synctoy to copy the library so that folder/file date and times are preserved rather than being stamped with the time the data was put on the pool drives. Part way through the copy I realized that I needed to turn on duplication for the main "MP3 Music" folder so I selected the main "MP3 Music" folder and said to duplicate the folder and all subfolders. So far so good, the remaining folders/files of the copy were duplicated in the pool as they were copied. The next day after the copy was complete I looked at my "MP3 Music" folder and found that the stuff copied before I turned on duplication still hadn't been duplicated. At this point I manually set duplication on for the main "MP3 Music" folder using the all subfolder to be included option. DB churned for a while, but did not duplicate the non duplicated stuff. I then went to each artist folder that still showed as not duplicated and set duplication for the artist folder. This works for all the album folders under each artist.
It appears that DB can't handle duplication when there's more than some number of folders/files greater than some amount (in my case it was about 12,000 songs across many artists/albums that needed duplication). Or was the problem something to do with turning on duplication for a folder while subfolders/files were being created in the top level folder?
I have open a support ticket.