Erase MBR from a Mac

Question

So I found a way to EFI install my Windows 8 to my MacBook Air - and now my partition table looks kinda ugly:

/dev/disk0
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *500.3 GB   disk0
   1:                        EFI                         209.7 MB   disk0s1
   2:                  Apple_HFS MacHD                   389.9 GB   disk0s2
   3:         Microsoft Reserved                         134.2 MB   disk0s3
   4:       Microsoft Basic Data LINUX                   50.0 GB    disk0s4
   5:       Microsoft Basic Data                         50.0 GB    disk0s5
   6:       Microsoft Basic Data SCHULE                  10.0 GB    disk0s6

So far so good...the problem is just, that I have a hybrid scheme - so I have GPT and MBR.

Over the time I have found gdisk to strip my MBR, or to pretty much erase it...which just didn't work, its still present:

Disk size is 977105060 sectors (465.9 GiB)
MBR disk identifier: 0x00000000
MBR partitions:

Number  Boot  Start Sector   End Sector   Status      Code
   1                     1    977105059   primary     0xEE

Now, when I try to boot windows 8, it shows me it's boot screen, and then just goes black and stops completely. My guess is, that it's doing the following:

• EFI "calls" Windows
• Windows performs preps for boot
• Next boot level appears, sees MBR, and tries to boot using it. <- Fails.

Since my MBR is nothing more but one big partition, I was very curios that it still attempted to boot from it. During the install, I was able to see my other partitions - outside of the 4-partition-limit - as well.

Looking at the windows drive, it appears to be a NTFS drive, and from what I know, EFI can't boot from such drives...however, it boots from Mac's HFS. So I am a little confused to what is really happening.

Any idea?

Answer 1

First, with a hybrid MBR, Windows will only install in BIOS mode. Thus, your claim that you began with an EFI-mode installation of Windows with a hybrid MBR is suspect; you've got to be wrong on one of those two points. If you really did have a hybrid MBR and you converted it to a protective MBR, Windows would have stopped booting. That's consistent with the symptoms you report, so my suspicion is that you were actually booting in BIOS mode with a hybrid MBR. If this analysis is correct, then the solution is to restore your hybrid MBR using gdisk, gptsync, or some other tool.

Second, the MBR data you presented is not for a hybrid MBR; it shows a valid protective MBR, which is a necessary part of a valid GPT. Thus, you should not attempt to remove the 0xEE partition. Doing so will render your GPT invalid. I'm not sure how the Mac's firmware will react to that.

Third, concerning EFI and NTFS, a Mac's EFI firmware includes drivers for both FAT and HFS+, and it can read boot loaders from either filesystem. For comparison, the firmware on a typical UEFI-based PC includes a FAT driver and that's it. Booting Windows from an NTFS drive on a UEFI-based PC is not a problem because an NTFS driver is included in the Windows boot loader file that's stored on the FAT EFI System Partition (ESP). The same thing can work on Macs, but getting Windows to boot in EFI mode on a Mac can be tricky. It's supposedly much easier with Windows 8 than with Windows 7, but I don't happen to have any references handy for how to do it. I get the impression that it's easier with some models than with others.

Answer 2

Try using Mac OS X's Disk Utility to do a repair of the whole disk (select the disk itself, not one of the indented partitions). Hopefully it'll see that you've screwed up your PMBR and recreate it. PMBR = Protective MBR: a PMBR tells old non-GPT-aware software that the whole disk is in use, so that the legacy software doesn't try to mess with the disk.

Your goal is to either have an unmolested PMBR, or an MBR that perfectly mirrors the GPT. You don't want to have GPT without any form of MBR.