Mac – Windows 10 won’t install in EFI Boot mode when hybrid partitions are present on Mac-Pro 5.1

efimac propartitionwindows

I am trying to install Windows 10 in EFI mode on a Mac Pro 5.1, to co-exist with Snow Leopard, also installed in EFI mode, in GPT partitions, and Windows XP, installed in BIOS mode, in an MBR partition.

So I set up hybrid partitions, using Gdisk, with 4 MBR partitions at the beginning of the disk (for some reason, the Windows XP installation will fail if the 4 MBR partitions are at the end of the disk, and for some other reason it needs to be on the 4th, but those are different issues).

Gdisk reports:

Everything looks OK to me.
But the EFI Boot Windows 10 installation doesn't see the GPT partitions, it only sees the MBR partitions and sees the GPT part of the disk as unallocated space:

And says: "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."

If there are no hybrid partitions, Windows 10 installs OK in EFI mode.

Is Windows 10 installation in EFI mode incompatible with hybrid partitions?

Has anyone encountered this or found a solution?

Best Answer

A Windows 10 installation in EFI mode is incompatible with hybrid partitions. A solution would be to install both Windows XP and Windows 10 in BIOS mode.

Related Solutions

Windows – Fixing a Windows partition boot using OS X tools

I'll post this as an answer, even though it didn't fix my problem – this was a lost case in the end. Aftermath: I dd'ed a backup of the Windows partition to my OS X partition:

sudo dd bs=512 if=/dev/disk0 of=windows_backup skip=732993496 count=173955112

Now I had a 80 gig hex dump which I could examine safely. I used a hex editor to search for all kinds of metadata:

"NTFS" (encoded in ASCII) which starts a NTFS volume.
"FILE" (encoded in ASCII) which starts a file entry record in the $MFT or master file table in NTFS.
NTFS system file names (like "MFT", tried with little-endian UTF-16 and ASCII)

... and so on. But without success. I was able to find all kinds of data from the dump, but all the metadata, including the boot record (the first sector of the volume), the $Boot file (the first 15 sectors after the boot record), the $MFT file which keeps record of all the files on the file system, were gone.

The thing that baffles me that I couldn't even find $MFTMirr file, which is the backup file of the metadata files and stored halfway of the volume.

I was able to find a backup of the boot sector from its standard location, the last sector of the volume. However, the data was old, from the era before resizing the volume. The boot sector has stored the offset of the $MFT file metadata file, but inspecting the references was moot, there was nothing of value there.

In the end, I concluded that the volume was totally borked. The moral of the story? Hybrid Master Boot Record is evil. Also, it seems that the program that resized the volume did a subpar job, failing to update some of the metadata.

Windows – Does Windows, running on latest Apple computers, see boot disk partitioned as MBR or GPT

If you install Windows with the Bootcamp Assistant from a standard Mac OS X system, Windows always will start from a PMBR/Hybrid MBR partition table embedded in the GUID partition table (Legacy MBR). MBR takes precedence in hybrid configuration even if a EFI boot might be possible.

If you use a non-Bootcamp Assistant EFI install method for Windows (e.g outlined here) Windows will boot in UEFI-mode (compatibility Windows 32bit/64bit) and you will find a * in the Gpt column for disk0.

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