"//{nas path}/Folder/" is no longer available

After several hours of attempting to move my music library (~ 20K tracks; 1,650 albums; 100GB) to a Buffalo LinkStation LS210D NAS, I have arrived at the following ‘conclusions’:

1. The error message “//[drive name]/music_folder is no longer available.” is non-specific at best, if not completely erroneous.   Upon successfully adding/updating a new, but relatively small, Music Library I can stream tracks from the NAS on my SONOS speakers simultaneously with the SONOS app displaying the error message; of course the music_folder IS available. 
2. There is some limit:   Current_#_Tracks_In_Library + #_New_Tracks_Added > N that triggers the error message.  Hence, if you attempt to add a large music library at once, you will exceed N immediately.   If you take an incremental approach, example add all albums beginning with the letter A, then B, etc. you will succeed until you get to a point where you exceed N; in my case the letter T. 
3. Once I have arrived at this threshold N, if I delete a few albums from Music_Folder, SONOS will successfully “Update Music Library Now”.  Yet when I copy the few albums back into Music_Folder, I get the error message again.   I did think it was possibly some syntax bug with a particular album folder, but I ruled that out by repeating the threshold test with various other album folders.
4. I think the SMB v1 or v2 may be a bona fide issue, however, it is tangential to the main issue here. 

The bottom line is a developer at SONOS should easily be able to troubleshoot and fix this bug as the problem is easy to reproduce, as evidenced by the number of us reporting similar experiences in this forum.