On Thu, Apr 7, 2022 at 5:21 PM Neal Gompa <ngomp...@gmail.com> wrote:
>
> On Thu, Apr 7, 2022 at 7:10 PM Demi Marie Obenour <demioben...@gmail.com> 
> wrote:

> > Can the disk be partitioned so that it is valid for *both* MBR and GPT?
> >
>
> There are two options here: protective MBR and hybrid MBR.
>
> Protective MBRs are well-supported and I believe created by default
> when you create a GPT partitioning scheme for BIOS systems already
> with Anaconda. I think it's done with UEFI as well, but if not, we can
> certainly force it.
>
> The second option is known as hybrid MBR. The gdisk utility can create
> them: https://www.rodsbooks.com/gdisk/hybrid.html
>
> Needless to say, it's... interesting... I'm not sure if more tools
> support hybrid MBRs since this was last refreshed in 2019.
>
> It's also kinda scary and not really necessary unless you need to
> interoperate with OSes that expect MBR in a mixed BIOS+UEFI boot
> setup.

Strictly speaking per the UEFI spec, hybrid MBR's don't exist.
Creating such a thing means it's really MBR and the GPT is invalid,
and it requires all software to treat it as such. The only valid MBR
with a valid GPT, is a protective MBR (PMBR) which is defined as
having one partition the size of the device or the maximum size
supported by MBR if the device is bigger.


-- 
Chris Murphy
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