Well, it looks like it's not hardware. I ran Memtest for a few hours to verify the ram is good. I changed the NIC. The problem persists. I installed Win 7 pro 32 bit version, and get the same errors. I monitored Task Manager to check memory usage, threads and handles. Memory use hovered around 550meg during the copy. No thread or handle leaks were observed. As a sanity check, I installed good old reliable XP and the problem goes away. So to summarize, it's not hardware. I don't suspect the NIC drivers are at fault ( I downloaded the latest Win 7 x64 driver set from the manufacturer) The IRPStackSize registry tweaks don't work. I'm not running IIS. It's just a simple folder copy of about 50 gigs from an XP machine on my LAN. The problem is reproducible on Windows 7, both 64 and 32 bit versions, but not on XP. Any ideas anyone? Thanks, Ken.C