On Thu, Apr 7, 2022 at 5:21 PM Neal Gompa ngompa13@gmail.com wrote:
On Thu, Apr 7, 2022 at 7:10 PM Demi Marie Obenour demiobenour@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.