Windows 8 installer complains about MBR partitions, but I don't see any?

Question

I would like to install Windows 8 on my iMac (mid 2010 model). Currently the main OS on the iMac is Ubuntu 14.04.

Using gparted I created an NTFS partition in preparation to install Windows 8 on it.

I created a bootable Windows 8 installation USB.

I booted the iMac from it.

Proceeded through the Windows 8 installation until it reached the section where you select the partition to which it should install.

I selected the partition that I had previously created through gparted and then received the error that in UEFI mode Windows 8 can only be installed to GPT partitions.

I tried clicking on "format" but the Windows 8 installer was not able to format that partition and frankly even if it could, I am not sure that this affects the partition table GPT/MBR ?

Not sure what to do at this point.

Update 1:

Here is the output of sudo gdisk -l /dev/sda:

GPT fdisk (gdisk) version 0.8.8

Partition table scan:
  MBR: hybrid
  BSD: not present
  APM: not present
  GPT: present

Found valid GPT with hybrid MBR; using GPT.
Disk /dev/sda: 1953525168 sectors, 931.5 GiB
Logical sector size: 512 bytes
Disk identifier (GUID): 21090AD7-A749-4E86-84E8-68CA38EDF40B
Partition table holds up to 128 entries
First usable sector is 34, last usable sector is 1953525134
Partitions will be aligned on 2048-sector boundaries
Total free space is 3437 sectors (1.7 MiB)

Number  Start (sector)    End (sector)  Size       Code  Name
   1            2048         1050623   512.0 MiB   EF00  
   2         1050624      1107335167   527.5 GiB   8300  
   3      1928376320      1953523711   12.0 GiB    8200  
   4      1107335168      1928376319   391.5 GiB   0700 

Answer 1

You haven't posted detailed partition table information, so I can't be sure of what's happening; however, it's almost certainly one of two things:

• Your disk uses the Master Boot Record (MBR) partitioning system and boots Ubuntu in BIOS/CSM/legacy mode. This is the safest way to install an Ubuntu-only system on a Mac, and it's what would result if you tried installing Ubuntu and the firmware happened to boot the installer in BIOS/CSM/legacy mode. If you then booted the Windows installer but the firmware made the choice to boot it in EFI mode, you'd see the error you describe. The solution is to force a BIOS-mode install of Windows. You might be able to do this by removing the EFI boot loader (which should be in the EFI/BOOT directory on your USB); or perhaps by holding down Alt or Option as you boot, you may be able to force a BIOS-mode boot.
• It could be that your hard disk has a hybrid MBR on the partition table. When it's fed a hybrid MBR, Windows interprets the disk as an MBR disk, ignoring the GPT data. When fed the same disk, OS X and Linux both treat the disk as a GPT disk, ignoring the MBR data (except to the extent that an MBR's type-0xEE partition is part of identifying it as a GPT disk). A hybrid MBR is the most common way of dual-booting OS X and earlier versions of Windows on a Mac. The problem with this hypothesis is that GParted will normally convert a hybrid MBR back to a conventional protective MBR, so it doesn't match your description. It's still a possibility if you omitted something, like running gptsync on the disk. If this is what's happening, then the preferred solution is to make some trivial change to the disk with GParted; or use gdisk to create a fresh protective MBR by using the n option on the experts' menu (x followed by n followed by w to save your changes). (You could go for a BIOS-mode Windows install using the hybrid MBRs, but as hybrid MBRs are a dangerous hack, it's better to install Windows 8 in EFI mode, if possible.)

To be certain which course of action to take, you must examine your partition table. This is pretty easily done with gdisk, as in:

$ sudo gdisk -l /dev/sda
GPT fdisk (gdisk) version 1.0.0

Partition table scan:
  MBR: protective
  BSD: not present
  APM: not present
  GPT: present

This example (which omits partition details) shows a GPT disk with a conventional protective MBR, as identified by the GPT: present and MBR: protective lines. If the MBR line reads MBR: hybrid, then that identifies a GPT disk with a hybrid MBR. If the MBR line reads MBR: MBR only and the GPT line reads GPT: Not present, then the disk is a straight-up MBR disk.

Answer 2

You must have formatted the disk with the MBR partition table. You might be able to do something to fix that without reformatting the drive, see comment below. You CAN however, backup your data, format with GPT, and install windows THEN recheck Linux.