Go read this:

https://discussions.apple.com/thread/4781793

It’s a discussion of why copies of large file from windows to macintosh fail, and includes a Windows registry change which fixes the issue. This was driving me nuts for months and had me on a set of wild goose chases (must be the router firmware….must be the mac since this doesn’t fail in other contexts … must be something in windows drivers … must have something to do with the drive mounted via usb where these files were usually headed, on and on). Anyway, 5 minutes after reading this my problem was gone.