

I’ve never tried to install windows onto an external device before, but I’d imagine something else is going on there. This could also be happening because your installing it on an external device.

The Apple update service - installed when you first run windows on bootcamped devices - contains registries for things on your computer that you wont get if you go right to 1809. All the drivers and support packages that are included in 1607 will not be installed on your Mac, and somewhere durring the process something went wrong (this is my guess). This might be happening because 1607 has native support for the 2017 MacBook pros hardware, and 1809 does not. You can find a link to download this here: I always download 1607 build and then download updates from there. Try installing an older Distr of windows.
