I recently wanted to test something on Windows, and not wanting to use a virtual machine, I decided to install Windows 8.1 on my MacBook.

All Intel based Macs use EFI with a GUID partition table (GPT), instead of the traditional Wintel BIOS/MBR combination. Until recently, this has meant that Windows can only be installed by emulating an MBR disk, typically by using Apple’s Boot Camp to create the Windows partition.

Since Windows 8 now supports installation under EFI1, and Boot Camp is fairly rigid about the placement of a Windows partition, I wanted to try installing Windows 8.1 using EFI and GPT just as I do with Mac OS X and Ubuntu.

Note: this procedure gets rapidly more complicated under only slightly unusual circumstances (such as having a non Windows OS installed, or more than one disk). Since this information involved some trial and error to discover, I post it here more for the record than as an endorsement of something that’s worth doing (it’s not too hard, but seriously, Ubuntu is free and much easier to install!).

Why the Windows install will fail

The issues below will prevent the installation of Windows 8 (and 8.1) from succeeding when installing under EFI on a Mac or other EFI based system. The lack of clear error messages actually makes an EFI installation seem impossible, despite the EFI boot loader being present on the Windows disc. However, the fixes are fairly straightforward once you understand the issues.

  • When installing from a USB key2, that key must use an MBR partition table
  • The target for the installation must use a GUID partition table (GPT)
  • The target disk must not have a hybrid GPT/MBR partition table, such as that created by Boot Camp, Disk Utility, or GParted3. A GPT with protective MBR is fine though, and protects the GUID partitions on unsupported systems
  • Windows will normally fail to install on systems with more than one GPT disk

A new Windows partition

The first task is to make some free space available, and then create a new NTFS partition for Windows to install to (size should be at least 20GB). It’s easier to do this now, before trying to install Windows, for reasons that will become apparent.

If you’ll be resizing any Mac HFS+ partitions then you should probably use Disk Utility on OS X. Likewise, for resizing ext3/4 and other file systems unknown to OS X, use GParted (booting from the gparted live cd if no Linux system is installed).

Note: On a Mac, the boot manager can be accessed by holding the alt key during startup.

Preparing a GUID Partition Table

You can check the GUID partition table (or create one from an existing MBR) using gdisk - which can be installed on Mac OS X or used from most Linux distros, such as the gparted live cd.

  1. From a terminal, run sudo gdisk /dev/<target-disk-id>4
  2. gdisk should display GPT: present, but must not list MBR: hybrid
  3. If the disk has a GPT without a hybrid MBR, quit by typing q then enter

If the target disk is listed with MBR: hybrid, then Windows will fail to install under EFI. The best way to correct the GPT is to replace the hybrid MBR using gdisk.

  1. Still in gdisk, use x to enter the expert menu
  2. Enter n to create a new protective MBR
  3. Check the protective5 MBR and GPT by using o and p respectively
  4. You can abort before writing any changes by using q to quit
  5. Once satisfied, write out the new table to disk by using w

If the target disk is listed with MBR: MBR only, then gdisk should have automatically created a GUID partition table in memory6. If the MBR partition table isn’t required to support older systems or legacy boot loaders, then use the menu commands shown above to check the existing partitions have been added correctly before writing out the tables to disk.

Install Windows to a GPT disk

Start the Windows installation using your preferred media, select the custom install option, then continue on to the partition selection screen and select the empty NTFS partition created previously. With a single non-hybrid GPT disk, Windows should install without complaint.

Note: While you can use the installer to format or delete existing partitions, using it to create a new partition from free space will likely create additional system partitions which you may prefer not to have littering your disk. If you don’t want to reboot at this point to create an NTFS partition, diskpart can be used to avoid the additional partition clutter.

With some free space ready to be partitioned, type Shift + F10 to bring up a command prompt and use the following commands.

# start the diskpart utility
diskpart

# select the target disk
list disk
sel disk <target disk>
list part

# create a new partition using the selected disk's free space
create part pri
format fs=ntfs label=Windows quick

# exit diskpart
exit

Manually mount the EFI partition

With more than one GPT disk, Windows will fail to install as it only expects to have one EFI system partition in which to store its boot configuration files. To work around this, use diskpart to manually mount the EFI partition from the target installation disk, then tell Windows to use that partition by using bcdedit.

Type Shift + F10 to bring up a command prompt.

# start diskpart
diskpart

# mount the efi volume
list vol
sel vol <efi volume>
assign letter=s
exit

# set the EFI partition with bcdedit
bcdedit /sysstore s:

Delete the previous boot configuration

If you’ve previously had Windows installed using EFI, then the previous boot loader and boot configuration data store (BCD) will likely still be installed in the EFI system partition. To prevent previous Windows entries from showing up as duplicates in the Windows boot manager, you can delete the previous BCD store before installing Windows.

Assuming the EFI partition is mounted to S:\, the following command will delete the BCD store.

# delete boot store from a previous windows install
del s:\EFI\Microsoft\Boot\BCD

If you forget to do this until Windows is installed, and a previous configuration did exist, then a system selection screen will be shown after each startup of Windows. To remove the additional entry, use bcdedit /delete {id-of-boot-entry}.

Continue installation of Windows

Back in the installer, you should now be able to select the new partition and continue the installation as normal. When Windows reboots, it may be necessary to use the system’s EFI boot manager to select the Windows partition and complete installation.

Why the Recovery Console will fail

With Windows installed, you may at some point need to use the recovery console. On EFI systems with more than one GPT disk, you’ll find that most of the recovery tools, such as System Restore, will fail due to the same issue that causes the installation to fail.

From Troubleshoot -> Advanced Options -> Command Prompt, you can manually mount the correct EFI partition using diskpart as described above. Once done, simply use the graphical menu to select System Restore or another recovery option.

Legacy boot manager

Windows 8 uses a graphical boot manager that’s only shown once the system is mostly booted. Apart from taking longer to switch between systems, there is no access to advanced boot options such as safe mode etc., without first being able to start the graphical boot manager or recovery environment.

Previously, advanced options could be accessed by holding F8 during startup, but Windows no longer checks for this key7 as it adds a second or so to the boot time. If you’d prefer to have the menu available instead, then the legacy boot menu policy can be enabled using bcdedit.

# allow F8 during boot to access advanced options
bcdedit /set bootmenupolicy legacy

Alternatively, the advanced options can be shown for the next boot only, or for every boot (without needing to hold F8).

# show advanced options for the next boot only
bcdedit /set onetimeadvancedoptions on

# always show the boot menu before system startup
bcdedit /set {bootmgr} displaybootmenu yes

  1. Windows 7 actually supports efi booting from CD, but the efi loader and related files were not included in the CD’s filesystem (there is a hack to work around this, but it doesn’t seem to work on my MacBook). ↩

  2. Write a new MBR partition table to a USB key and add a single FAT32 partition. Then copy the Windows install files from the disc to the USB key using rsync -aPh <source>/. <key>. ↩

  3. Disk Utility only adds a hybrid MBR when creating a new partition table and will retain any protective MBR already present. Annoyingly, GParted will re-add a hybrid MBR whenever it modifies a disk that uses GPT, and this will prevent Windows from starting under EFI until the hybrid MBR is removed. ↩

  4. On Linux systems, the disk will normally be named using the form sda, where the a represents the first disk. On Mac OS X, the form is disk0, where the 0 represents the first disk. ↩

  5. A protective MBR usually lists only one partition, that spans the entire disk to prevent non-GPT aware tools from identifying the disk as being uninitialised or empty. A hybrid MBR exploits the protective MBR to make up to four primary partitions available to systems that don’t support GPT disks. The use of a hybrid MBR is discouraged, and not supported by Windows when booting under EFI. ↩

  6. It may be that gdisk can’t write out the GPT due to lack of free spaceat the start of the disk. In this case you can try to resize the first partition to start a few megabytes later. ↩

  7. The check is supposedly still there, but is so short that it can’t be activated. ↩