Installing Windows 8.1 on a 2013 Mac Pro

Installing Windows 8.1 on a 2013 Mac Pro was challenging, but I’ve managed to get it working after a lot of hassle. I’ll share my experience and the solution that worked for me.

I was using the Mac Pro and the Promise Pegasus2 R4. I would divide the internal SSD to get Windows installed on that one (installing Windows directly on the Pegasus2 is not possible).

After running the Bootcamp assistant, i let it create a USB drive with the windows image and the support files. Then I used the wizard to divide the partition.

After rebooting into the Windows installer, it got stuck at partitioning with the error “Couldn’t create new partition or locate existing one“. I then made the horrible mistake of deleting the partition and creating a new one from the windows installer, which broke the partition table. Luckily I could fix it with the diskpart CLI utility in OS X (it replaced the windows partition with 3 mini-partitions that couldn’t be removed in the old-fashioned way).

Apparently the error message was caused because the installer got confused with the Thunderbolt storage attached. Got rid of that error by disconnecting everything except the display, mouse, keyboard and USB drive containing the Windows installer (bare minimum)

After trying the Windows installation again, it ran fine until the last stage before the reboot. It came with the error message that it had an error installing the boot sector. It rolled back the installation. Upon rebooting, the Windows drive was shown by holding down the Option key. But choosing the new Windows partition froze the system.

I then used Disk Utility in OS X to create a Windows partition manually. Then ran the installer again. But then the partitioner in Windows Setup gave the error “Windows cannot be installed on this disk. The selected disk has an MBR partition table. On EFI systems, Windows can only installed on GPT disks“.

Side note: Windows 7 and 8 in x64 support EFI. Normally if you install Windows on a Mac and use the installation DVD, it boots into regular BIOS mode, thus can be installed on an MBR partition. I tried the same, but since the Mac Pro doesn’t have an optical drive I had to use an external drive. And apparently the Mac boots external optical drives in EFI mode too. The Bootcamp wizard is aware of this, and creates a GPT partition on a non-superdrive Mac but an MBR partition on a superdrive Mac.

After looking for a solution, I had something that worked. So I used the Bootcamp wizard again and ran the Windows installer from the USB drive as I did as my first step. During the Windows installation, it complained that the partition was not in NTFS. So I used the format option (not the delete and create option!!). The installer then failed with the boot installation error message.

I rebooted the system into the new Windows installation. It froze the Mac, as expected. Then I turned it off, unplugged the USB drive and did a PRAM reset (boot while holding Command + Option + P + R until it reboots). Then I plugged the USB drive back in and started the Windows installation again by booting the USB drive (hold down the Option key while booting, and select the USB drive). I did not format the partition and it installed all the way to the end.

After the first reboot (hold down the Option key and select the new Windows partitiion at each reboot), I got a BSOD. After a reboot I got through to the next step of the installation. While configuring the last stage, it froze at the “Almost Ready” screen. Forced a reboot and I got Windows up and running!

Then I installed the Bootcamp drivers and the Pegasus2 drivers, reconnected everything and that was finally it!

So to summarize the steps (TL;DR):

  1. Unplug everything except for the bare essentials to install Windows (display, mouse, keyboard, USB drive)
  2. Run the Bootcamp wizard to prep the USB drive and to partition the SSD for Windows.
  3. When the installation starts, select the Bootcamp partition. If it complains about not being NTFS formatted, choose the Format option and click on Next to continue.
  4. If you see an error message about the installation of the boot sector, reboot into the new Windows partition (will freeze)
  5. Turn off the Mac and unplug the USB drive.
  6. Perform a PRAM reset (hold down Command + Option + P + R while turning on the Mac until it reboots)
  7. Re-connect the USB drive and boot from it (hold down the Option key while booting and then select the USB drive).
  8. Run the installer again, but don’t format the partition.
  9. After each reboot, hold down the Option key and manually select the new Windows partition.
  10. It might cause a BSOD or freeze, but a reboot should fix that (BSOD during 1st boot, stuck at configuring at 2nd boot).
  11. Install the Bootcamp drivers from the Bootcamp folder on the USB drive.

Note: If the boot error message remains after the 2nd try or if you’re unable to perform the PRAM, you can also try this (not tested, requires the Windows 8.1 installation disc and an external optical drive):

  1. When the error message during the installation appears, insert the Windows 8.1 installation disc. Unplug the USB drive and reboot into the Windows 8.1 installation disc (hold down Option key during reboot and select the optical drive).
  2. Instead of clicking on Install Now, click on Repair my computer
  3. Select Command Prompt (through Advanced Options)
  4. Enter these commands:
    bootrec /fixmbr
    bootrec /fixboot
    bootrec /scanos
    bootrec /rebuildbcd
  5. Plug the USB drive back in
  6. Boot into the new Windows partition
  7. It should continue with the installation. If it freezes, it didn’t work unfortunately.

These steps are for the 2013 Mac Pro, but might also apply to other non-optical drive Macs like the Macbook Air. This worked for me, but might not work for you. Make sure to backup your important files before you attempt this.

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