How to say no to all "do you want to overwrite" prompts in a batch file copy?

Here's a workaround. If you want to copy everything from A that does not already exist in B:

Copy A to a new directory C. Copy B to C, overwriting anything that overlaps with A. Copy C to B.

Unless there's a scenario where you'd not want to copy existing files in the source that have changed since the last copy, why not use XCOPY with /D without specifying a date?

echo "No" | copy/-Y c:\source c:\Dest\

You can make a text file with a single long line of "n" then run your command and put < nc.txt after it. I did this to copy over 145,000 instances where "No overwrite" was what I wanted and it worked fine this way.

Or you can just hold the n key down with something, but that takes longer than using the < to pipe it in.