I live in a house divided. I use both Mac and Windows, but my spouse is strictly Windows. Recently, I upgraded her aging PC with a new video card for better Windows 10 support. All seemed to go well — until I realized I’d booted the system but not plugged in the old PS2-style keyboard. The simple act of attaching the keyboard made the PC shut down and refuse to restart.
Because none of my repair attempts worked, and it would be some days before I could purchase a new machine, I took a flyer and attached the PC’s hard drive to my Mac Mini via a StarTech SATA/IDE-to-USB 3.0 adapter (more info). I then held down the Option key during startup to select the boot drive.
To my great surprise, the Win10 system not only booted, but ran relatively well. My spouse was able to access her files and applications until we were able to pick up a new laptop (at Costco). For better or worse, she decided to stick with Windows. (For professional and personal reasons, I stayed neutral in the decision.)
The temporary setup indicated that it was running under Microsoft’s Windows To Go technology (more info), which I understood only worked with the Enterprise version of Windows. I can’t say whether my setup violates the license agreement for a consumer version of Win10. I used it as a temporary solution. But it might be a way to boot a Windows system that you need to use occasionally.