I am a Mac user who needs access to a windows program. Instead of using bootcamp (which I have used in the past) I would like to put windows on one partition of an external hard drive so I don't have to take up space on my computer. What I am after is being able to boot into windows from one partition and leave the other partition free to use for extra storage (on mac or windows, like a flash drive). Here is the tutorial I followed, but when I go to boot into windows on my mac it says "Unable to verify startup disk".
Does anyone know how to resolve this?
Note: I did do this a couple years ago with my old mac and it worked, and I don't remember what tutorial I used. So it is possible, at least with macOS from a couple years ago
EDIT: This is regarding a 16" MacBook Pro running macOS catalina