Disk Activity Stops and System Eventually Freezes

With heavy(?) disk use, my system appears to stop disk activity. Resource Monitor shows nothing under Overview and Disk where there would be e.g. system and explore.exe listed as images before this condition. I'm trying to copy 100-200GB from one disk to another. After some time, this condition happens but I can still to other things. The network is up, I can e.g. ping my LinkSys, ssh to...