Simultaneous EFI and Legacy bootloader installation

[Mario Limonciello]

On 03/30/2016 09:48 AM, Lennart Sorensen wrote:
> On Wed, Mar 30, 2016 at 09:16:35AM -0500, Mario Limonciello wrote:
>> Can you comment what version of Windows you had noticed this behavior? 
>> We actually factory install as old as Windows 7 with GPT disks in legacy
>> mode at Dell.  We don't factory install Windows 8 or Windows 10 in
>> legacy mode.
> Well certainly with Windows 7 and Windows 10, if you boot legacy mode,
> you don't get an option to use GPT.  And everything I have ever read
> says you can't do that with Windows because Microsoft says so.
> Certainly the hybrid partition table option is a problem since Linux
> and BSD prioritize GPT while windows prioritizes MBR.
>
> Every answer I can find to how to boot windows on GPT with legacy involves
> using an MBR USB key or a floppy as the boot media with the main install
> on GPT (but in that case it is in fact NOT booting from GPT).
In the public consumer facing installer it sounds like this option
doesn't populate automatically.  We don't use the same installer for
factory install in Dell.  We also only do GPT disks with Win7 legacy
when circumstances require it (large disks).

> I can't find anything anywhere that says you can boot on a legacy system
> from GPT with Windows 7, 8, 8.1 or 10.  I can only find lots of people
> asking how to do it and being told you can't without a separate boot
> device that isn't GPT.
The way to do it is to boot a USB disk with WinPE manually and format
the disk with diskpart ahead of time.  You might also be able to
accomplish this with the advanced options that can get you to the
diskpart tool before you actually select the partitioning page.

Windows should happily install onto the existing partitioning then.
>> Shouldn't it be possible to install GRUB into the partition boot record
>> (PBR) of the ESP?
> Certainly when I have used grub to run legacy systems with GPT due to
> large disks, I have used the BIOS Boot partition and grub has been happy,
> while without it you need to use the block map mode that grub highly
> discourages because it is as fragile as lilo always was.  The BIOS Boot
> partition contains the stage2 of grub raw, it does not have a filesystem
> unlike the ESP.  To put stage2 on the ESP you would have to then use
> the block map mode which defeats the purpose of using it and you might
> as well just block map the file in /boot.  Since you don't want to use
> the block map mode, the BIOS Boot Partition is the better option when
> you want grub on a legacy system with GPT.
>
> The < 500 bytes in the PBR is way too small for stage2 of grub and no
> better than using the MBR area (which works for a GPT disk too).
> It could only hold stage1.
Thanks for explaining, this makes sense to me now.
>
>> Yes, this scenario is why I was recommending in legacy mode to install
>> the removable path bootloader (\efi\boot\boot$ARCH.efi) by default.