If you have the Windows Support Software for your model Mac and a Windows 10 ISO file downloaded from Microsoft, then you can create a bootable Windows 10 USB flash drive installer without using the Boot Camp Assistant. The steps are given below.
- Use Disk Utility to erase and ExFAT format the USB flash drive. Choose the Master Boot Record Scheme.
- Mount the Windows 10 ISO file and the use the Finder application to copy all the files to the flash drive.
- Use the Finder application to copy the Window Support Software to the flash drive. If you do this correctly, then the
BootCampfolders should appear in the root folder of the ExFat formatted volume on the flash drive. For you model Mac, there should also be an
AutoUnattend.xmlfile in the root folder of the ExFat formatted volume on the flash drive. If you are not sure the Windows Support Software is for your Mac, then use the Boot Camp Assistant to download the files.
If you use the Mac Startup Manager, then you should choose the icon labeled Windows to BIOS boot from the flash drive. The message below occurred because you EFI booted from the flash drive.
“Windows cannot be install to this disk. The selected disk has an MBR partition table. On EFI systems, Windows can only be installed to GPT disks”
The message below occurred because the partition labeled BOOTCAMP was not NTFS formatted.
“Windows cannot be install to this hard disk space. Windows must be installed to a partition formatted as NTFS”
You can NTFS format the partition labeled BOOTCAMP by highlighting the partition, then selecting the Format button. The BOOTCAMP label will disappear after formatting. This BOOTCAMP label will reappear later after installing Boot Camp.
Finally, I should point out that not all releases of Windows 10 have been compatible with every Mac model that Apple officially states are capable of running Windows 10. If you are have problems with installing or using a certain release, the try a different release.