The copy of a movie from the pc to the NAS stops in the middle and i have to reset the pc. Space on Nas more than 100 GB.

A Western Digital My Book World Edition 1 T   Model: WDH1NC10000

Thats a bit vague… how is the NAS connected to your network? The PC? Do you have other PCs on the network? Can they transfer files? If you are wireless what kind of security? If wired have you tried replacing your ethernet cables?