Ok folks, this is very strange.
DNS-327L with 1.03 firmware.
Attached to the USB: WD MyBook 1TB with NTFS file system.
Sharing options: it's not possible to define a network share on the USB disk, so it is accessible to everyone on the network.
This is what happens:
I can view the USB disk and all its files. However, I cannot access SOME OF THEM.
Example: two .avi files in the same folder, same extension and similar dimension. One is accessible (via web browser, network folder, vlc player or WDTV), the other gives the following error:
web browser returns an error 500;
copy via file system access gives an I/O error;
VLC logs say file system error;
WDTV says it can play the selected file.
The files are all accessible if I plug the WD directly to the PC or to the WDTV.