I am trying to install Windows via Boot Camp Assistant and get the following message:

The startup disk cannot be partitioned or restored to a single partition. The startup disk must be formatted as a single Mac OS Extended (Journaled) volume or already partitioned by Boot Camp Assistant for installing Windows.

In Disk Utility only one unpartitioned drive is displayed.

In Terminal, the output of distutil list is:

/dev/disk0 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *1.0 TB     disk0
   1:                        EFI EFI                     209.7 MB   disk0s1
   2:                  Apple_HFS Install macOS High S... 90.9 GB    disk0s2
   3:                  Apple_HFS bup macpro              364.0 GB   disk0s3
   4:                  Apple_HFS Untitled                544.7 GB   disk0s4

/dev/disk1 (internal, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *4.0 TB     disk1
   1:                        EFI EFI                     209.7 MB   disk1s1
   2:                  Apple_HFS Macintosh HD            4.0 TB     disk1s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk1s3

/dev/disk2 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.1 GB   disk2
   1:                        EFI EFI                     209.7 MB   disk2s1
   2:                  Apple_HFS Crucial 500gb           499.2 GB   disk2s2
   3:                 Apple_Boot Recovery HD             650.0 MB   disk2s3

Disk0 is used for TM backup. Disk1 is main HD, where Windows is to be installed. After Windows is installed I intend to format and clone disk2 from disk1 and install it in the Mac as the boot drive. I only installed HS and network settings on disk2 to confirm the partitions would be set up.

The question is, how do I merge/delete the partitions on disk1 to satisfy Boot Camp Assistant?

  • 1
    Have you confirmed that disk1 is using a 4096 byte sector size? If the the drive is using the legacy 512 byte sectors size, then you can not install Windows in the last 2TB of the drive. In other words, if the 4TB drive has a 512 byte sector size, then the Boot Camp Assistant is not going to let you install Windows on the drive. You can determine the sector size by entering the command diskutil info disk1 | grep Size. Sep 10, 2018 at 19:24
  • Terminal command reply Disk Size: 1.0 TB (1000170586112 Bytes) (exactly 1953458176 512-Byte-Units) Device Block Size: 512 Bytes
    – EMF
    Sep 10, 2018 at 22:03
  • If you want to install Windows to this 4 TB drive, you probably will need to do this manually. Which version of Windows did you want to install? Is Windows 32 or 64 bit? Does your optical drive work? Sep 10, 2018 at 23:21
  • 64 Bit. Optical drive works. I was planning on installing Win 10, which i believe is not compatible with 5.1, but there is work around. But if we can back up. If my Internal drive is not set up incorrectly, why is BCA preventing the install. I thought the solution would be to boot from an external source. Format and do a clean install on on disk1 and transfer settings etc using Migration Assistant. However I did this to disk2 and got the same partitions. Is there a way to clean install and change the partitions? What would happen if I used terminal to merge the partitions on disk1 into one?
    – EMF
    Sep 11, 2018 at 5:40

1 Answer 1


Below is a response to your statement: "If my Internal drive is not set up incorrectly, why is BCA preventing the install."

Apple does no officially support Windows 10 on your Mac, but Microsoft does. Say you first installed Windows 7 with Apple's Window Support Software. Now, if you upgraded to Windows 10, then Windows 10 would be using the Windows 7 drivers provided by Apple. So, there is no reason why you can not install Windows 10 with Apple's Windows Support Software written originally for Windows 7.

Below is an answer to the question: "If my Internal drive is not set up incorrectly, why is BCA preventing the install?"

The Boot Camp Assistant installs Windows at the end of a drive. This is physically impossible to do with your disk1. The sector numbers for your 4 TB drive are to large to be stored in a legacy MBR partition table used to BIOS boot Windows.

Furthermore, you probably would have to remove disk0 in order to install Windows on your Mac. This requirement was published in early editions of the Boot Camp Installation & Setup Guide. For example, the paragraph below was taken from Boot Camp - Installation & Setup Guide (Mountain Lion) Mar 14, 2013.

If you have a Mac Pro with more than one internal disk and you want to install Boot Camp on a disk that isn’t in the first hard drive bay, remove the drives in the lower- numbered bays. You can reinstall the drives after you install Boot Camp.

I do not think I understood the rest of the questions you posted in your latest comment.

I would try installing Windows without using the Boot Camp Assistant. The first step would be to burn the Windows ISO file to a DVD. You will also need a flash drive for the Windows Support Software from Apple. You can omit the DVD and use the same flash drive to hold the Window ISO files, but this makes the installation procedure more complex.

  • Many thanks. Disk0 is only a USB connection so no problem. If I installed the SSD in the Mac Pro as the boot disk and removed Disk0 would that work? Am I correct in assuming that the disk name is attached by the OS at boot up and not fixed? The question I am trying to ask is should I install the SSD in the first bay or doesn't it matter? Thanks again.
    – EMF
    Sep 11, 2018 at 11:52
  • I have never seen any post here a Ask Different where disk0 was an external USB drive. I do find it odd that you encounter this. Your best chance for success would be to put the SSD in the first bay. The mac should then assign this drive as disk0. The drives are assigned disk numbers based on the order the drives are encountered. For example, you can restart macOS and find the external drives have changed numbering. This is also true for the partition numbering. Sep 12, 2018 at 2:43
  • I tried to repartition the drive to 4 partitions (inc 2 hidden) with a total of 2.2 TB between them, but the terminal command I used to show all partitions in Disk Utility didn't work in HS. I got the same BCA pop up. I removed the external drive and then installed SSD in bay 1, 4tb in 2 and two other drives in 3 and 4. Bay one was made boot drive and was assigned disk0 by OS. BCA was happy to proceed? Many thanks again.
    – EMF
    Sep 13, 2018 at 6:23

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