Remote into your NAS via telnet/ssh and use the mv <source> <target> & command, replace <source>/<target> will the actual location. The ampersand (&) at the end will launch it in the background. In another word, there's no need to maintain a active remote session and you can off your PC.
FYI, I and tried and timed it before, moving 100GB of data across volume took around an hour.