Re: Use grub-rescue on a non-bootable RAID-formatted drive [WAS: Re: status: how to install grub to non-bootable RAID-formatted drive?]

["Alexander V. Makartsev" <avbetev@gmail.com>]

On 1/31/26 11:28, David wrote:

> ...
> On Thu, 29 Jan 2026 at 11:34, Alexander V. Makartsev
> <avbetev@gmail.com> wrote: [h]
>
> > I imagine you could type at grub rescue prompt something like this:
> > grub> set pager=1
> > grub> insmod normal
> > grub> insmod part_msdos
> > grub> insmod diskfilter
> > grub> insmod mdraid1x
> > grub> insmod ext2
> > grub> ls
> > (proc) (hd0) (hd0,msdos1) (hd0,msdos2) (md/0)
> > grub> set prefix=(md/0)/boot/grub
> > grub> set root=(md/0)
> > grub> linux ($root)/boot/vmlinuz-6.12.63+deb13-amd64 root=/dev/md126
> > grub> initrd ($root)/boot/initrd.img-6.12.63+deb13-amd64
> > grub> boot
> >       
> It won't work because 'grub>' prompt is not the same as 'grub rescue>'
> prompt. grub-rescue will have appeared *because* grub cannot find any
> "third portion" (including modules normal, part_msdos, diskfilter,
> mdraid1x, ext2, and others) so trying to 'insmod' them will fail, as David
> Wright explained:
>     
    You don't know what you are talking about. This is not the
    "chicken-egg problem".
    "insmod" commands will work in 'grub>' AND 'grub rescue>' and
    essential modules are provided by grub's core image.
    "insmod normal" will fail and return "error: disk 'mduuid/...' not
    found.", this was expected, because grub needs prefix to be
    available to load it.
    However, all essential modules, like "part_msdos, diskfilter,
    mdraid1x, ext2" are available without prefix and in fact preloaded
    at boot time.
    Grub's "ls" command outputs "(hd0) (hd0,msdos1) (hd0,msdos2) and
    (md/0)" because these modules are preloaded.
    This is the reason why I called these commands "redundant" in my
    messages.
    I've seen OP's reply about "ls (md/0) outputs Error: unknown
    filesystem" which is strange, it should output "(md/0): Filesystem
    is ext2.".
    
    The problem OP having is something else, as I've written before, but
    he seems refusing to cooperate and provide more information, or is
    too busy to work on it.
    He seems to ignore the other possibilities that could interfere with
    booting process, like old BIOS and too modern 3TB disk, MBR
    partitioned 3TB disk, Advanced Format disk, etc.
    
    Since we already reinstalled grub properly and that didn't fix the
    problem, next steps should be fixing broken array "/dev/md127" and
    checking filesystem on "/dev/md126" with "fsck.ext4".
    But before that he needs to make a good backup of his data, because
    it becomes increasingly too risky to continue, as I told him before.
    I've also told him other possible options like adding another disk
    less than 2TB in size into PC, if he has it handy, and move "/boot"
    to it to use it as a boot disk.
    Or repurpose his dysfunctional swap on "/dev/md127" into a plain
    partition with ext3 filesystem and move "/boot" to it.
    These options I would pursue if I had the same problem.

--

 With kindest regards, Alexander.

 Debian - The universal operating system
 https://www.debian.org