Tried that too as SMB performance to CPU cost is not great on the Pi. ftp is timing out as well but only on the USB mounted drive. I can copy files from the SD card to the target machine easlly. I uninstalled Kaspersky to see if that was the issue but no dice.SMB gives random issues. SFTP (or just ftp) is much better. Most sync apps have the option for it and there are plenty of server apps available too. Also find it more reliable as you can see the log and find issues.
I even set up a ftp site on the Windows box with IIS but same thing happened there when I tried to ftp using command line.