Re: GRUB2 (packaged with Ubuntu 12.10) won't boot OS X

[Chris Murphy]

On Nov 8, 2012, at 12:09 AM, Alex Kanavin <address@hidden> wrote:

> 2012/11/8 Chris Murphy <address@hidden>:
> 
>> Well that's debatable, because it's inherently fragile to depend on NVRAM 
>> for this. "Zapping the PRAM" is a common (if not sometimes mystical)
>> troubleshooting technique on Macs, and the instant you do this your computer 
>> cannot be made to revert to the behavior you now have. So it's not only
>> fragile, but it's obscured, and also it's basically broken your ability to 
>> boot the primary OS: OS X.
> 
> Why can't it be reverted? If Ubuntu's installation used efibootmgr to
> write to NVRAM, so can I, via some recovery USB stick:

Yeah if you remember all of that or saved it somewhere. And the use of 
efibootmgr is beyond obscure. You're basically depending on something fairly 
user hostile and not reproducible without special knowledge.


> What I'm trying to troubleshoot here is
> GRUB's inability to boot OS X either through the generated script
> (that does some magic with OS X kernel) or through chainloading, which
> does seem like a GRUB's bug.

Well, you may need to build GRUB2 from current source. Anyone else is going to 
ask you to do that first anyway just because so many EFI related bugs have been 
fixed recently.

>> Weird. The Windows option is hard wired and requires three things to be 
>> true: the first 440 bytes of the MBR contain boot loader code, the MBR 
>> contains more than one entry, one of the entries other than the first entry 
>> in the MBR has an active flag set.
>> 
>> So something has done those three things. The Windows options is the gateway 
>> to the Apple EFI activating the CSM-BIOS, so that would seem to indicate 
>> you're booting CSM-BIOS, but the fact it's a dead end with "no operating 
>> system" rather than going to GRUB possibly means you're booting EFI, in 
>> which case why do you have a hybrid MBR, and code in the MBR bootloader 
>> region.
>> 
>> What do you get for:
>> 
>> dmesg | grep -i efi
> 
> That is attached.

You are EFI booted. You might find out what created that inhospitable hybrid 
MBR. Oh I bet you prepared the drive using Boot Camp Assistant first, didn't 
you? If you did and don't also have Windows on the computer, you might use 
gdisk to create a new protective MBR, and write out a new GPT. The hybrid MBR 
is fraught with problems and technically right now you have a stale and invalid 
GPT.

Chris Murphy