Use BiBM to boot from GPT BIOS boot partition

[auryx]

Hi all,

Hope you are well. I haven't played around with my BiBM in years and now am trying to do something new and failing. Can you let me know how (and if!) it's possible?

I have a system which boots in Legacy mode (i.e. I don't use UEFI). I have BiBM installed on one disk with an EMBR (which is the primary boot disk, in terms of BIOS order) and typically, I only use it to boot to Windows 10, on a different disk which is configured as MBR.

I have now installed Fedora Linux on a 4TB on a third disk. That disk is configured as GPT. The Fedora installer has created the usual soup of Linux partitions (root, home, boot, etc.) but also has created a "BIOS boot partition" to enable GRUB2 to boot Fedora with Legacy boot, even though the disk is GPT.

If I change my boot order on system startup to boot from this 4TB drive, it works fine. Legacy boot loads GRUB2 and I select Fedora, no issues. I now want to boot that Fedora install from my BiBM menu.

If I use the Boot Edit menu in BiBM to add an entry for the Fedora disk and try to save it, I get an error message about the boot partition having to be registered in the MBR (or similar - apologies, I forgot to write down/snapshot the error). This is regardless of whether I select the overall disk, or the "GPT Reserved" first partition, to try and target with the boot menu entry. Examining the disk using "Partition Work" shows no partitions have the "bootable:yes" entry.

Any ideas what I am doing wrong? Apologies, my level of knowledge is low/old/rusty so the question may be a stupid one or badly explained. In short, I guess I want to use BiBM to chainload GRUB2 on the BIOS boot partition of my GPT drive.

Thanks

Mike.

[Bob Coleman]

I held off answering hoping you'd get a more authoritative answer, but I don't think BIBM is compatible with GPT disks.

[auryx]

Thanks anyway!

I wondered if that was the case. I'm nearly sure I used it to boot an OS from a large disk before, but maybe I had that partitioned into chunks and used MBR instead.....can't remember.

Oh well! Never mind Thanks Bob.

auryx

[Bob Coleman]

I don't think a "large disk" is the issue. It's more likely because GPT and MBR are entirely different partitioning schemes.

[Brian K]

Mike,

If your 4 TB HD has 4K native sectors you can make it a MBR disk.

[CyberSimian]

Bob Coleman wrote:
> I don't think a "large disk" is the issue.

My understanding is that an MBR disk is limited to a maximum size of 2 TB. So the original poster could partition his 4 TB disk as MBR, but only the first 2 TB would be accessible -- the other 2 TB would be inaccessible.

I note that Brian K has said that 4 KB sectors allow a 4 TB disk to be partitoned as MBR. I shall be interested to see if this can be made to work.

-- from CyberSimian in the UK

[TeraByte Support]

4K visible sector size brings up the 2TiB limit by 8x because the normal
sector size is 512 bytes (and MBR supports a value up to 4GiB sector count)

On 5/29/2020 1:53 AM, CyberSimian wrote:
> Bob Coleman wrote:
>> I don't think a "large disk" is the issue.
>
> My understanding is that an MBR disk is limited to a maximum size of 2 TB. So the original poster could partition his 4 TB disk as MBR, but only the first 2 TB would be accessible -- the other 2 TB would be inaccessible.
>
> I note that Brian K has said that 4 KB sectors allow a 4 TB disk to be partitoned as MBR. I shall be interested to see if this can be made to work.
>
> -- from CyberSimian in the UK
>
>

[Brian K]

Some calculations...

A MBR disk can have a maximum of 2^32 sectors

2^32 = 4294967296 sectors

With 512 byte sectors

4294967296 * 512 = 2199023255552 bytes

2199023255552 bytes = 2 TiB (2.2 TB)

But if you have 4K native sectors instead of 0.5K sectors, a MBR disk can still have 4294967296 sectors with...

4294967296 * 4096 = 17592186044416 bytes

17592186044416 bytes = 16 TiB (17.6 TB)

[auryx]

Hey everyone,

Sorry for the delayed reply, I hadn't checked the forum in a few days. Thanks for the suggestion Brian - that's an intriguing possibility! I need to work out (a) how to check if my 4TB disk has 4K native sectors (b) if that "works" with my OS (c) if it means it can be MBR. I'll have a look.

The other weird thing is that I am nearly 100% sure that I have used BiBM to chain-boot (via GRUB2) an OS on a GPT-formatted disk before. I know BiBM doesn't work if your system uses UEFI boot, but mine boots in Legacy ("BIOS") mode.

I'm beginning to suspect that what is "throwing" the BiBM config is the dedicated BIOS Boot Partition that the newer version of Fedora has created, because those didn't exist (or weren't used, as far as I know) a few years ago. I wonder...if I can somehow install GRUB2 and bootloader code to a "standard" partition on my GPT drive, make that the /boot partition, and then remove the BIOS Boot Partition completely, and point BiBM to the new "standard" /boot partition instead.......anyway, just thinking aloud. Might be a stupid idea. I'll try both these ideas when I have a moment!

Will let you know...

Thanks again,

Mike.

[Bob Coleman]

I THINK BIBM doesn't know how to create a Boot Item to boot a partition of a GPT disk, but I could be wrong.