Got this from a friends blog, mention of using esutil to copy large files;
The full article is here;
It mentions that eseutil is a better option for moving large files, yet someone on the above mentioned they tried to copy a 2GB file and it was slower. On the grand scheme of things I would not consider a 2GB file a large file, anything over 50GB is a large file. Migrating data from an old server to a new server generally means min file size of 10GB and normal 'Drive' backup files of 40 to 80GB at time.
I've never seen Exchange fail to move its database file from one drive to another, so I'm hopeful that tomorrow I'll try out using eseutil to move a 40GB file off a NAS onto a server and see what happens.