I had a similar problem.

This might possibly help you.
I own a Western Digital Worldbook NAS. In the process of copying large files (i.e. pictures) from my laptop to the NAS, I noticed that images got mangled. I called Tech Support and they told me that that type of NAS has problems accepting mass transfer including either too many samll files or a small number of large files.
In a nutshell, music files copy OK but pictures do not make it unscathed.
You drive might have the same problem.