Presumably you have run a "Scan Disk" from the front controls of the B2.
If not please do so and look again.
Finally if you have been moving files via NAS, then it will most likely have worked - but unless you have to screens of Windows File Explorer open side by side, and you can move things very carefully, you can sometimes make a slip.
In terms of "properties" of the PC/Mac directory and the B2's music folder how big in terms of size and number of files is the discrepancy between the two?
Fred