Some backgroundTo install Pop!_OS I had to reconfigure the BIOS to disable the Intel RST setting and switch from UEFI boot to Legacy. Windows 10 and the BIOS updater only run under the UEFI system.
The Windows 10 InstallHaving rebuilt quite a few Windows 10 machines I have experience with installing Windows 10 from a USB flash drive. However, what I was after this time is to install and run Windows from an external drive. The planned target device was a 500G 7200rpm 2.5" HDD that was put into a USB 3.0 enclosure. To do this work, I used a Lenovo Thinkpad Yoga S1 running Windows 10. I was able to follow an article to image Windows 10 using Rufus. Here is an article that explained how to target an external USB 3.0 HDD with Rufus. So, key settings were to specify the HDD, use the Windows 10 ISO, Windows to go installation, Windows Home selected, and GPT/UEFI format.
Once Rufus finished its work, I copied over the BIOS update file to the external drive.