Do not use Microsoft's Media Creation Tool. It will refuse to install to an external drive. Rufus is the hero here.
Download the free, open-source Rufus tool, which is essential for creating the "Windows To Go" image. Step-by-Step Installation Guide install windows to external ssd
However, simply copying Windows files to a drive won't work. The standard Windows installer blocks installation onto USB drives by default. You have to trick the system. Do not use Microsoft's Media Creation Tool
Here is a step-by-step guide to installing Windows 10 or Windows 11 onto an external SSD. open-source Rufus tool
Windows treats an external drive installation just like a normal internal installation.
Before starting, ensure you have the following hardware and files ready: