Usually setting up a dual-boot for Windows on a Mac is a fairly straightforward procedure—use Boot Camp Assistant to create the USB installer and then have Boot Camp Assistant help you set up the partitions and install Windows.
We ran across a rather odd scenario with some Mac Pros, whose internal storage we'd upgraded from the built-in storage to the Aura SSD. The SSD uses the PCIe interface but seems to use that as an intermediary for the larger SSD attachment, which Mac OS X sees as an external drive, and Bootcamp does not like to install to an external drive.
This may not be the only approach that works, but it seems to have worked for us:
- Set up your .iso and USB stick as you normally would in Mac OS X using Boot Camp Assistant. If you get an error that says You cannot partition an external disk, a RAID disk or an internal disk on which FileVault encryption is in progress, ignore it. Yes, you're using an external disk, but right now you're not partitioning. You're just setting up the USB stick.
- Use Disk Utility to make a second partition that's MAC OS (journaled). (Don't make a FAT32—when I did, the Windows installer complained.)
- Keep the USB stick plugged in.
- Download rEFInd, and after you extract it, run sudo /Users/username/Downloads/refind-bin-versionnumber/refind-installNote: this command won't work in El Capitan unless you disable SIP first.
- Reboot and pick the Windows installer from the list of bootable drives. You'll likely see three options (Boot Fallback boot loader from NAMEOFUSB, Boot Windows from whole disk volume, and Boot Windows from NAMOFUSB). If you try the latter two, you'll get Error: Not Found while loading legacy loader. So, instead, you want to select Boot Fallback boot loader from NAMEOFUSB.
- When you get to the Where do you want to install Windows? part of Windows Setup, you'll see your Mac OS (Journaled) partition with a warning like Windows can't be installed on drive 0 partition 4. (Show details). If you click on the details, it'll say Windows cannot be installed to this hard disk space. Windows must be installed to a partition formatted as NTFS. Windows cannot be installed to this hard disk space. The partition is of an unrecognized type. In other situations (internal drive, Windows 7), I've been able to get around this by formatting the partition as FAT32 in advance (but when I did that with an "external" drive and Windows 10, I got an error of Windows cannot be installed to this disk. The selected disk has an MBR partition table. On EFI systems, Windows can only be installed to GPT disks). So select the partition (e.g., Drive 0 Partition 4) and then click Delete. Then click on the Drive 0 Unallocated Space that freed up, and then click New and then Apply. You'll see that Windows Setup created a small 16 MB "MSR (Reserved)" partition and then another, much larger (NTFS) partition.
- During installation, you may get an error like this: If you do, just let it go through. I had that happen a couple of times on one Mac, once on another, and not at all on a third. They seem to be fine. I'll update this post if that proves not to be the case.
- Windows should install fine after that. If you get a weird situation in which Windows boots up okay but hangs when rebooting, install all Windows updates and Boot Camp updates as well.