How do I install Windows 10 on a Mac Mini that is booted from an external SSD?
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.