How are files routed through the network when using a copy command with UNC paths as the source and destination?

Question

Given a command like robocopy "\\UNC1" "\\UNC2" /E /MT:128 /R:0 /W:0 executed from a workstation.

Do files move like this

or more like

Answer 1

The latter.

Robocopy reads a file into a local buffer and then writes that buffer to another file. (Of course, it only reads and writes part of a larger file and repeats in a loop until finished.) If a file is accessed on another host via UNC then the file access uses the network.

To optimize the network flow you need to run robocopy on one of the servers involved.