How do I install Windows 10 on a Mac Mini that is booted from an external SSD? I have a Mac Mini 2018 that is booted from a 2TB external SSD. I would like to install Windows on the internal 256 GB SSD. Preferably using the entire internal disk. When I run Boot Camp  Assistant it complains External storage device Attached. The two alternatives that I have considered:


Run Windows 10 in a VM - I've done this in the past but was not satisfied with the experience.
Boot from the internal drive and install Windows 10 on a partition there. - This would be a waste of the limited space on the drive.


I take Apple at its word that I can't resize the Windows partition after I create it. I don't expect to use Windows a lot, but when I do, I want it to behave itself. Can the process be completed without Boot Camp Assistant?
The primary reason you can not resize the Windows partition is because the Boot Camp Assistant installs Windows at the end of the drive. If you do not use the Boot Camp Assistant, then you could place the Windows installation files in a ExFAT formatted partition at the end of the drive. Once Windows is installed, you can delete this partition and expand the main Windows partition to reclaim the space. Or, avoid the problem entirely and install Windows to the internal drive by booting the Windows installer from a flash drive.

Below are the steps for creating a flash drive installer.


Erase the flash drive. Use a ExFAT format with a MBR partitioning scheme.
Mount the Windows ISO and copy the files to the flash drive.
Use the Boot Camp Assistant to download the Windows Support Software.
Copy the Windows Support Software to the flash drive.


You will have to configure you Mac to allow booting from a flash drive. Once booted from the flash drive, I am not sure how far you can get using the GUI. You may need to use the CLI to complete the installation. I do know the driver used access the internal drive is not loaded until you reach the window asking for the product key.