On large copy operations it is always good to have the possibility to resume after the process has been interrupted. And also check the state of the parts that have already been transfered.
I suggest you use a different tool for this.
One easy soulution would be to download the free evaluation copy of TotalCommander http://www.ghisler.com/download.htm and use its "Synchronise Directories" feature. I have used it a lot for moving and copying mp3s around the LAN or between partitions and it is really easy and safe:
It allows you to have 2 windows side by siode and compare the already copied files and continue copying the rest. At the end you can do a size based check or even a content based check (check the "by content" option), to see if everything went fine. I suggest doing the content check only once in the very end, because it could take some time.
I use this gui based tool a lot on such occasions because it can handle interruptions, errors and because at the end you can be sure all has been copied 1:1
Other possible tools are the command line based "robocopy" from microsoft. There is also the commandline based "rsync" which is very good at such tasks, but it is from the linux world and I have preferred robocopy over it on windows.
There are other gui tools like Beyond Compare or WinMerge, here is a list:
If you still think you are having a network problem afterwards, let me say I think it is very hard to track down a network related problem like yours without having real access to the machine. Could be anything. You could reduce the setup step by step and see at which point the problem disappears (Remove other machines from the lan or switch them off, remove switches and cables between the 2 machines and replace them with cables that work, switch off and/or uninstall the firewalls (be careful then) and other additional software on both machines one by one. In the end connect the machines directly or over a router that you know works. You can exchnge the network cards or add additional network cards and use those to find out if a broken nic is the source of the error. You could also boot the one or the other system from a live CD like ubuntu or knoppix live cd and try heavy copy operations with them. They can integrate into your existing LAN.
Remember to always only change ony part of your setup and repeat your tests until yoou know if the error is still there. So you can encircle and track down the source of the problem.)