How do I update Windows 10 that is installed on an external hard disk?

Solution 1:

From Microsoft article, it mentioned “Older Windows 8 or Windows 8.1 Windows To Go workspaces cannot be upgraded to Windows 10 workspaces, nor can Windows 10 Windows To Go workspaces be upgraded to future versions of Windows 10. For new versions, the workspace needs to be re-imaged with a fresh image of Windows.”

Here is link: https://docs.microsoft.com/en-us/windows/deployment/planning/windows-to-go-overview#a-href-idbkmk-wtgdifadifferences-between-windows-to-go-and-a-typical-installation-of-windows

But I found a similar question, check if you could modify the registry key PortableOperatingSystem value from “1” to "0" to do the upgrade.

Here is link. Windows To Go (Windows 10) Upgrade to anniversary update? https://social.technet.microsoft.com/Forums/windows/en-US/9f96f44c-4c14-4a51-87c9-7123a0ad29f5/windows-to-go-windows-10-upgrade-to-anniversary-update?forum=win10itprosetup