Then a colleague pointed me to the "copy /z" command:
/Z Copies networked files in restartable mode.
And this simple, built-in tool turned out to be the thing I needed. It was, every time, able to restart the download properly, and after a few times, it downloaded the file correctly.
As you may know, I am a reluctant recent switcher to Windows, coming from Linux and Mac OS X. I confess I still have a condescending attitude to everything in Windows, but this teaches me a lesson: it pays to know the native tools of the system you are using. I had subconsciously dismissed the built-in DOS copy command as too primitive to work, but it turned out to have the capabilities I needed.
A couple of tips:
- If you interrupt the command with Ctrl-C, it will delete the portion copied so far and you'll have to restart the copy from the beginning. If you must interrupt a copy, simply terminate the network connection (it will die with an error, but leave the copied-so-far file) or close the window where the command is running.
- When you restart it and it asks if you want to overwrite, you must answer "yes". You can also use the /Y flag.
