bootcampexternal-diskpartitionwindows
I have followed this guide: https://9to5mac.com/2017/08/31/how-windows-10-mac-boot-camp-external-drive-video/
When I select EFI Boot from my Samsung M3 portable drive it reboots to the boot recovery assistant and asks for my pw to unlock the macOS disk.
This is the second HD I've tried this on, both to the same effect.
The drive is partitioned 75/25 Time Machine / Windows.
Any help is appreciated
When using the Winclone product to install Windows on a external USB drive, it is extremely important that you first read all the instructions provided by Winclone.
Winclone provides the Web page: Running Windows from an External Drive. This page provides links to two videos. The first link is the same as given in your question. The second link provides the answer to your posted question.
The general rule is that the 2015 and newer Macs can no longer boot operating systems using the BIOS method. This is the method used in the installation instructions you refer to. Also, generally 2011 and older Macs can not boot Windows from external drives. Therefore, the installation method, you refer to, generally only works for the 2012 through 2014 model year Mac computers.
The steps to determine if your Mac can BIOS boot operation systems is given below.
From the menu bar, select "About this Mac". The pop up window should display the model year and screen size of your Mac. The pop up window for my Mac is shown below. I have a 20-inch, Mid 2007 iMac.
But even if both computers qualify to BIOS boot Windows from external drives, you have the problem of the drivers. One of the steps, in the instruction you refer to, is to install the Boot Camp Support Software. This software installs the correct drives for your model Mac. If you then try to boot Windows using a different model Mac, the installed drivers will be wrong for the this Mac.
Best Answer
When Boot Camp installs Windows 10 on a Mac, any Apple supplied drivers required to boot Windows 10 are automatically included in the driver store during the installation process. There is no outward indication that this takes place. After Windows 10 finishing installing, software including additional drivers are installed. This process is very evident due to output shown on the display.
When watching the video, I did not see any step where the Apple supplied drivers, required to boot Windows 10, were installed to the drivers store. This means the instructions supplied in this video are incomplete.
So you may ask: Why did the example shown the video work? My best guess would be that Windows 10 was installed on an older Mac. Microsoft makes an effort to include in the ISO all the drivers needed to boot an installation of Windows 10 for any Windows 10 compatible computer. For an older Mac, there would be no needed to copy the Apple supplied drivers to drive store, because the required files would already be in the store.
Since your Mac is a 2018 model, the drivers needed to boot may not have been included yet in any Windows 10 ISO file. If this is the case, Windows 10 will not be able to boot.
My best advice would be to not try to use third party software in an effort to trick Windows 10 into booting from an external drive. All the software needed to install to an external drive is already included in the Window 10 ISO file.
If you want to proceed based on the video, then eliminate the use of the VMWare Fusion software. In other words, use Boot Camp to install Windows 10 to your internal drive, then use WinToUSB to transfer to the external drive. You then can remove Windows 10 from the internal drive.