MacPro Mid 2010 - High Sierra - Can’t Install Windows 8.1 via Boot Camp Assistant

Per the title, I’m trying to install Windows 8.1 via BCA, and I always encounter this issue: when BCA restarts into the Windows installer and asks which drive I want to install windows on, it throws an “Windows cannot be installed on to this disk. The selected disk is of the GPT partition style.”

I have tried clicking the advanced options there and click Partition, but I still get the same message after it appears to partition the drive.

For specs, I have:

• BCA version 6.1.0 (6067.50.6)
• macOS High Sierra version 10.13.4
• Mac Pro (Mid 2010)
• 4 x 2 TB SATA Disks (all native - no custom ones put in)

For the walkthrough of the process I’ve been trying:

1. Open BCA
2. Proceed with both options selected (Download support software and install windows 7 or 8) - it prompts to have a USB external drive plugged in to put the support software on (I do so)
3. After downloading support software, it will throw an alert asking for an installer disk if I don’t have a DVD-RW with the Windows 8.1 ISO burned onto it - I do so
4. I select Bay 2‘s harddrive and have the option selected to “Erase disk and create a single partition for Windows” and hit Continue to trigger a restart
5. After a long restart into the Windows 8 installer, I hit Next at the first screen with the defaults (language, time & currency, keyboard method)
6. I click “Install Now” on the next screen (the only option besides Repair your computer)
7. I enter my Windows 8 product key and click Next
8. I click “I accept the license terms” checkbox and click Next on the license agreement screen
9. I click “Custom: Install Windows only (advanced)”
10. On the “Where do you want to install Windows?” screen, I have the following options:
    • Drive 0 Partition 1: EFI
    • Drive 0 Partition 2: BOOTCAMP (Windows can’t be installed on drive 0 partition 2–>)

      Windows cannot be installed to this disk. The selected disk is of the GPT partition style. Windows cannot be installed to this hard disk space. Windows must be installed to a partition formatted as NTFS.“

    • Drive 1 Partition 1: EFI
    • Drive 1 Partition 2
    • Drive 1 Unallocated Space
    • Drive 2 Partition 1: EFI
    • Drive 2 Partition 2
    • Drive 2 Partition 3
    • Drive 3 Partition 1: EFI
    • Drive 4 Partition 1: SEPDRV (this is the USB external drive with the BCA drivers and support stuff)
11. I select “Drive 0 Partition 2: BOOTCAMP” and click “Format”—> OK.
12. It now shows “Drive 0 Partition 2” (no BOOTCAMP) and still has “Windows can’t be installed on drive 0 partition 2” except this time the detail is just “Windows cannot be installed to this disk. The selected disk is of the GPT partition style.”

I’ve tried resetting SMC and NVRAM before starting this process as well just in case, with no change.