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Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop

Cross-posted to 4 groups.

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On 2025-10-31 12:56, Java Jive wrote:
>
> As per subject, as I now have on the same PC some OSs  -  Ubuntu24, 
> Windows7/10  -  installed on an MBR disk and Windows 11 installed on a 
> GPT/UEFI disk, I'd like if possible to find a way of booting any of 
> them, and MBR/GPT USB sticks, without constantly having to change the 
> BIOS settings on the PC.
> 
> Research into whether this is possible seems to produce mixed results, 
> mostly not, but a few suggesting it's possible, but I haven't yet found 
> anything with clear and understandable instructions on how to achieve it.
> 
> My own gut feeling is that it should be possible, but, while I have a 
> good understanding of how a PC boots an MBR disk, I have less 
> understanding how one boots a GPT/UEFI disk.
> 
> Has anyone here been able to achieve this?  Does anyone know of good 
> reliable sources of information about this?

Thanks for the earlier responses, I have now some success to report ...

You may recall the disk layout I was trying to achieve success with:

Disk 1:  256GB (nominal) SSD - MBR partitioning
     P1:  Win  7 Pro, NTFS
     P2:  Win 10 Pro, NTFS
     P3:  Win  7 32-Bit Pro, NTFS
             (for old scanner with only 32-bit drivers)
     P4:  Ubuntu 24, ext4

Disk 2:  2TB (nominal) HD - MBR partitioning
     P1:  Windows Data, NTFS
     P2:  Linux Data, ext4

Disk 3:  128GB (nominal) MiniSSD - GPT partitioning
     P1:  128MB UEFI Boot, FAT32
     P2:  Win 11 Pro, NTFS

Before I started this work, when booted via MBR and Disk 1, Grub gave 
access to all the OSs except W11P, while when booted via UEFI/GPT and 
Disk 3, only W11P could be booted, whereas obviously I wanted to have 
just one method of booting every OS.

Therefore the question was, should I try to find a way of booting W11P 
via UEFI from the legacy Grub installation on D1, which would have 
required some sort of forward compatibility of the legacy MBR 
installation, or should I attempt to load all the other OSs via UEFI via 
a new Grub installation on D3, which would require backward 
compatibility?  I felt the chances of forward compatibility were less 
than the chances of backward compatibility, so, despite the extra work 
involved, and certainly it was a great deal of work, I chose the latter.

However, as it has turned out, I've only been able to achieve partial 
backward compatibility in that all the 64-bit OSs can now be booted from 
D3, but not the 32-bit OS, I've not been able to find a way of booting 
Win 7 Pro 32-Bit from UEFI.  Further, I have not been able to find a way 
of booting *ANY* 32-Bit OS, not even a UEFI boot Win 8 Pro 32-Bit 
installation USB, when using UEFI on that particular PC (so probably 
none of the others either, because they're all identical or nearly so), 
so I suspect that this is a firmware limitation with this range of PCs.

FTR, this is how I got the other 64-Bit OSs to boot from Grub on the GPT 
disk, even though they themselves are on an MBR disk ...

1)  I renamed MS' D3P1:/Boot folder to 'boot', ie all lower case.

2)  I copied the previous MBR grub installation from D1P4:/boot/grub to 
D3P1:/boot/grub.

3)  Similarly, I copied the UEFI grub installation from an Ubuntu 24 
installation USB over the previously copied files in D3P1:/boot/grub.

4)  In the PC's firmware, I set up a new boot option named 'Grub' to 
boot ...
	D3P1:/EFI/Boot/grubx64.efi

As as result I was now able to boot into Grub displaying the previous 
menu from the MBR disk (because in step 2 I'd copied the grub.cfg across 
with everything else and taken steps to prevent it being lost in step 
3), but of course most of the options didn't work.

However, the existing Ubuntu 24 installation was able to boot from this 
new Grub menu, the only discernible difference being that an additional 
initial message is displayed briefly, it reads ...

EFI stub: Loaded initrd from LINUX_EFI_INITRD_MEDIA_GUID device path

... and thereafter the boot proceeds apparently as normal.

5) Next, I thought it should be possible somehow to boot via UEFI into 
Ubuntu and simply run 'update-grub' to complete the setup, as per this 
advice ...

https://askubuntu.com/questions/831216/how-can-i-reinstall-grub-to-the-efi-partition

... but this proved a complete waste of time.  First I tried doing it 
from the existing Ubuntu 24 installation; when booted via UEFI, the EFI 
system is available, but when doing a chroot to perform update-grub, the 
EFI subsystem and its associated variables were lost.  So next I tried 
booting via EFI from an Ubuntu 24 installation USB, but the same thing 
happened.  Finally, after much machination I installed a separate UEFI 
Ubuntu 24 on a spare disk and booted from that, and finally succeeded in 
getting the EFI subsystem available within the chroot, but update-grub 
didn't find the other OSs on the MBR disk anyway, although it did now 
complete without error and so create a working entry for the W11P 
installation on the GPT disk, which reads (beware unintended line-wrap):

5a) menuentry 'Windows Boot Manager (on /dev/sdc1)' --class windows 
--class os $menuentry_id_option 'osprober-efi-1043-DB71' {
	savedefault
	insmod part_gpt
	insmod fat
	set root='hd2,gpt1'
	if [ x$feature_platform_search_hint = xy ]; then
	  search --no-floppy --fs-uuid --set=root --hint-bios=hd2,gpt1 
--hint-efi=hd2,gpt1 --hint-baremetal=ahci2,gpt1 1043-DB71
	else
	  search --no-floppy --fs-uuid --set=root 1043-DB71
	fi
	chainloader /EFI/Microsoft/Boot/bootmgfw.efi
}

But for the others I had to wing it, using little clues from various 
researches online, none of which gave a complete template or revealed a 
crucial extra step to get Windows 7 & 10 to boot, which is that you need 
to load the module 'ntfscomp'  -  I guess comp is short for 'compatible' 
or some related word.  Also, you can't use drivemap but anyway don't 
need to, and you have to change the 'chainloader' command, like so ...

6)  Change a current MBR menu item of this form ...

menuentry "Windows ... {
     ...
     insmod ntfs
     ...
     drivemap ...
     chainloader +1
}

... to a GPT equivalent of this form ...

menuentry "Windows ... {
     ...
     insmod ntfs
     insmod ntfscomp
     ...
     chainloader /Windows/Boot/EFI/bootmgfw.efi
}

7)  The next thing to be done is to create the BCD files within the 
relevant Windows installation(s).  This can be done by booting into a PE 
environment, most probably from an installation media and then choosing 
Repair ... Command Console.

7a) First check the drive letters associated with the various partitions 
on the various disks, and adjust the one you want to work with to be C:

     DISKPART
     LIST VOL

If a different volume than the one you want to work with is C:, first 
temporarily assign it a different letter, say T:, then assign the volume 
you want to work with, here assumed to be 2, to C: ...

     SELECT VOL 1
     ASSIGN LETTER=T:
     SELECT VOL 2
     ASSIGN LETTER=C:
     EXIT

7b) Then, back in the console, give the following command ...

     BCDBoot C:\Windows /s C: /f ALL

... the 'ALL' parameter of which instructs BCDBoot to copy new BCD files 
into both ...

     C:\Boot\

... and (you might have to create the following parent directory chains 
before running the above command) ...

     C:\EFI\Microsoft\Boot\
     C:\EFI\Microsoft\Recovery\

Repeat 7a and 7b for any other Windows installations.

When complete, though I'm not sure that it's actually necessary, on the 
principle of being better safe than sorry I use DISKPART to reassign the 
drive letters back to what originally they were.

Then reboot, and it all works.

It's annoying about the 32-Bit problem, though of course I can still 
boot it via MBR  -  I need to use that for one particular old scanner 
for which I cannot get 64-bit drivers; it's useful to keep it going 
because it's quite suited to old gritty material which might scratch the 
glass of the new one, and also it's the only one I have with an 
Automatic Document Feeder.

Perhaps next I might try seeing if I can boot W11P from the original MBR 
menu, which would achieve the desired state of only needing one boot menu.

-- 

Fake news kills!

I may be contacted via the contact address given on my website: 
www.macfh.co.uk

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Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-10-31 12:56 +0000
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop "Carlos E.R." <robin_listas@es.invalid> - 2025-10-31 14:26 +0100
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Windows Elf <windows.elf@outlook.com.invalid> - 2025-10-31 14:57 +0000
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Paul <nospam@needed.invalid> - 2025-10-31 12:04 -0400
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Theo <theom+news@chiark.greenend.org.uk> - 2025-10-31 16:06 +0000
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-11-02 14:12 +0000
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-11-11 00:27 +0000
    Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Paul <nospam@needed.invalid> - 2025-11-10 22:45 -0500
      Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-11-11 13:19 +0000
        Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Paul <nospam@needed.invalid> - 2025-11-11 10:13 -0500
          Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop "Carlos E.R." <robin_listas@es.invalid> - 2025-11-14 13:24 +0100
            Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Paul <nospam@needed.invalid> - 2025-11-14 13:03 -0500
              Re: scanners. "Carlos E.R." <robin_listas@es.invalid> - 2025-11-14 20:31 +0100
                Re: scanners. Paul <nospam@needed.invalid> - 2025-11-14 17:52 -0500
                Re: scanners. "Carlos E.R." <robin_listas@es.invalid> - 2025-11-15 04:06 +0100
        Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-11-15 01:57 +0000
          Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop "Carlos E.R." <robin_listas@es.invalid> - 2025-11-15 14:51 +0100
            Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-11-15 15:01 +0000
              Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Paul <nospam@needed.invalid> - 2025-11-15 11:54 -0500
        Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2026-02-20 17:44 +0000
          Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Paul <nospam@needed.invalid> - 2026-02-20 13:46 -0500
            Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2026-02-20 20:01 +0000
              Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2026-02-20 23:29 +0000
    Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop TJ <TJ@noneofyour.business> - 2025-11-16 13:58 -0500
    Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Java Jive <java@evij.com.invalid> - 2025-12-16 17:04 +0000
      Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop "Carlos E.R." <robin_listas@es.invalid> - 2025-12-16 21:27 +0100
  Re: Is it possible to dual-boot both MBR & GPT without changing BIOS settings on laptop Adrian Caspersz <email@here.invalid> - 2025-11-22 18:22 +0000

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