bootcamp – Making a second Windows installation on macOS Catalina

During 2012 model year, Apple started shipping iMacs without Optical drives. Also, Apple started transitioning to (U)EFI booting of Windows. However, your Mac was designed to legacy BIOS boot all Windows installations.

Note: This method of installation may require temporary use of a wired keyboard and/or wired mouse.

Use macOS Catalina to create a Windows 10 installation flash drive.

To test this answer, the Windows 10 iso file Win10_22H2_English_x64v1.iso was downloaded from this Microsoft website. For a Late 2012 iMac, the Windows 10 install disk can be created by the Boot Camp Assistant. Below is a snapshot of contents of this flash drive.

Create a partition for the second Windows installation.

When testing, the Disk Utility application included with Catalina (macOS 10.15.7) had a tendency to fail, when attempting to shrink the Apple_APFS partition and add a new ExFAT formatted Microsoft Basic Data partition. Therefore, this answer recommends using the diskutil command to accomplish this task. For example, to shrink a 499.0 GB Apple_APFS partition to approximately 300 GB and make a new Windows named partition of about 199 GB, the following command would need to be entered.

diskutil apfs resizecontainer disk0s2 300G exfat Windows 0

Note: This command will leave disk0 hybrid partitioned.

Make Windows the default to boot and boot to Windows.

This can be done by either of two ways given below.

• From the Startup Disk pane of the System Preferences application.
• By holding down the control key while selecting Windows from the Mac Startup Manager.

BIOS boot from the flash drive.

Restart the Mac, then hold down the option key until the Mac Startup Manager icons appear. Boot from the flash drive by selecting the external (or flash) drive icon labeled Windows.

Install Windows 10.

Proceed with the installation until a display similar to the one shown below appears.

1. Select the partition named Drive 0 Partition 3: Windows.
2. Format the partition. The Windows label will be removed. The display should now appear similar to the one shown below.

3. Procced with the installation of Windows to this partition.