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Espruino is a JavaScript interpreter for low-power Microcontrollers. This site is both a support community for Espruino and a place to share what you are working on.
@MichaelS
Just posting that I also had this problem (on Windows 10). The key observation is the FS version of the virtual COM port does not appear (since it's not detected, whereas another port is detected when using the board normally). If you look in device manger, you can see an error icon next to the COM port when trying to flash.
So I tried the driver that Gordon suggested, and reporting that the solution is correct - but you do need to pay attention to the driver installation instructions. It's not enough to install the installer, but (from the readme file), you do need to go into the installation directory and run another installer (in my case Win8/dpinst_amd64.exe) and then it was ok.
Thanks again Gordon.