Re: Disk Utility repartition destroys hybrid MBR, Windows no longer boots
Re: Disk Utility repartition destroys hybrid MBR, Windows no longer boots
- Subject: Re: Disk Utility repartition destroys hybrid MBR, Windows no longer boots
- From: Chris Murphy <email@hidden>
- Date: Tue, 11 Jun 2013 14:36:55 -0400
On Jun 2, 2013, at 7:29 PM, Thomas Tempelmann <email@hidden> wrote:
>
> You can create a MBR for booting Windows (Bootcamp) that is listing
> different partitions than what the GUID partition table (GPT) lists.
>
> For instance, you can have 6 partitions in the GPT, yet still boot
> into boot camp by listing the relevent Windows partition in the MBR.
>
> That works because the firmware boot code looks at the MBR for booting
> windows, and so that Windows, while OSX looks at the GPT, even if the
> MBR is not GPT comform (the MBR of a GPT scheme should contain only
> one entry, thereby pointing to the GPT).
I can confirm this does work with all of the Apple EFI firmwares I've tested it with, but that's a rather tiny number compared to the total models with EFI64. One problem is these hybrid MBRs are violations of the GPT portion of the UEFI spec. But the much bigger problem is that different utilities will interpret such a partition scheme differently depending on whether they choose to honor the MBR exclusively, the GPT exclusively, or have another method to resolve the inherent ambiguity with two unsync'd partition tables.
This is exactly what's getting users into trouble when they successfully resize an OS X partition on 10.6.x, which lacks a Recovery HD partition, therefore the hybrid MBR is left intact by diskutil. (On 10.7 and 10.8, the hybrid MBR is replaced with a PMBR when the GPT contains 5 or more entries. The presence of a PMBR renders Windows unbootable.) And then when the user reboots Windows and resizes the Windows volume, that utility only alters the hybrid MBR entry. And then upon OS X reboot, the Windows volume no longer appears because its entry in the GPT is wrong. In some cases I've seen OS X Disk Utility incorrectly interpret the volume as FAT32 and try to repair it.
If the user is sufficiently warned to avoid making partition modifications with any other tool, except the one that created the hybrid MBR in the first place, then it might be useful. But the majority case is this significantly increases the risk for data loss.
https://discussions.apple.com/message/22219007?ac_cid=tw123456#22219007
https://discussions.apple.com/thread/5087650?start=0&tstart=0
The better solution Apple should have come up with by now is supporting UEFI/GPT booting for Windows, rather than depending on the EFI CSM (BIOS) + MBR approach in use now. It's inherently risky depending on hybrid MBRs.
Chris Murphy
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