On Thu, 29 Jan 2026 at 18:27, D. R. Evans <[email protected]> wrote:
> All right, let's see where we are now, on this grub-rescue sub-thread. Welcome back. > I will elide much of your (extremely useful/helpful) commentary, just to > make this e-mail less overwhelming. > > Following several attempts to use a separate drive/CD/USB stick to get > things working (so far without success), the current situation if I take > everything else out, and just leave the RAID drive in the machine, and > boot to the "grub rescue>" prompt, I now see (I have to type everything > manually into this e-mail; I'll obviously try to be really careful about > that): > ls: > > (hd0) (hd0, msdos2) (hd0, msdos1) (md/0) (fd0) error: failure reading > sector 0xb30 from 'fd0' Great, having only the one drive in the machine is exactly what we need to see. > The fd error is, I'm sure, irrelevant. I agree. > The actual RAID filesystem from which we want to boot is the second > partition on the hard drive: (hd0, msdos2). Ok. > set: [in this output I'll use "<UUID>" to mean a UUID that it prints, which > starts with "8d86" and ends with "0aed"] > > prefix='(mduuid/<UUID>)/boot/grub' > root='mduuid/<UUID>' > > Grub looks for its third portion using the "prefix" value that > > 'grub-install' wrote into the second portion. That's what you see when > > typing 'set' at the 'grub-rescue' prompt. You're seeing the > > 'grub-rescue' prompt because that stored value of "prefix" is wrong, > > and the second portion of grub is telling you that it can't find the > > third portion. > > That paragraph _really_ helped my understanding; it made the GNU > documentation a lot clearer. Great. > I note that your link to [grm], https://olinux.net/grub-rescue-mode/, > returns a 521 "Web server is down" error (at least, it has done so every > time I've tried it, so I have been unable to look at that information). Don't worry about it. That was just a simple demo of some basic grub-rescue use with no details specifically useful to you, so nothing there was important. > > So now that we are sure that you understand tab-completion, you can use > > grub-rescue's tab-completion in combination with its 'ls' command to > > explore your disk's filesystem with the aim of discovering where the > > grub third portion files are. You would do this by typing something > > like: > > > > grub rescue> ls (hd0)/ > > > > and then using tab-completion to see what grub finds in the root > > directory of your hard drive. > Perhaps I'm not understand something. I tried typing each of the > following in turn at the "grub rescue>" prompt: > > ls (hd0)/<TAB><TAB> > ls (hd0, msdos2)/<TAB><TAB> > ls (hd0, msdos1)/<TAB><TAB> > ls (md/0)/<TAB><TAB> > > None of them did anything at all. > > You had written before that "Occasionally tab-completion does not work in > grub-rescue. But it usually does,". Perhaps this is one of those > occasions where it does not :-( I agree. Looks like tab-completion doesn't work there. That's not a showstopper, because tab-completion is just a convenience feature. You can use 'ls' to do the same work, it is just slightly more fiddly. > It seemed like, from what you said, that one of the following _should_ > work: > > prefix=(X)/boot/grub > root=X > insmod normal > normal > > where X is one of: hd0 > hd0,msdos2 > hd0,msdos1 > md/0 > most likely either hd0,msdos2 or md/0 being the correct value. I agree. > I tried them all, and at the "insmod normal" step, they all returned: > "error: unknown filesystem". Ok, but that's trying too much at once. What I would do is to take each of the devices that you reported above that your plain 'ls' found, namely (hd0) (hd0, msdos2) (hd0, msdos1) (md/0) (fd0) and I would ask grub-rescue to show us what content if anything it is able to read in the root directory of each device in turn. Can you please report the result of these interactive grub-rescue commands, after booting your system with only the drive being rescued connnected: ls (hd0)/ ls (hd0, msdos1)/ ls (hd0, msdos2)/ ls (md/0)/ Hopefully one or more of those will be readable by grub. If so, the next step will be to use the directory names which that result showed, allowing you to run another longer 'ls' command to explore the readable content of those directory names, with the aim of finding your grub module files (third portion) somewhere. > Just to be absolutely certain that the RAID disk hasn't somehow been > compromised, I then reattached a drive with a bootable OS, booted into > that OS, and made sure that I could still mount the second partition on > the RAID drive; it still looks good, with the entire "/" hierarchy still > in place, including /boot. Ok good. > My simple brain says that if I can access <partition 2 on the RAID > drive>/boot from a running OS, we just haven't found the magic > incantation yet for doing the same for setting the value of prefix and > root in grub rescue. I agree. We are close to finding out which if any of your partitions grub is able to read and hopefully find the files it needs to continue booting. On Thu, 29 Jan 2026 at 18:39, D. R. Evans <[email protected]> wrote: > D. R. Evans wrote on 1/29/26 11:27 AM: > > You had written before that "Occasionally tab-completion does not work > > in grub-rescue. But it usually does,". Perhaps this is one of those > > occasions where it does not :-( > > > > FYI, I just found this: > > https://superuser.com/questions/1237684/how-to-boot-from-grub-shell says: > > > When you’re at the grub> prompt, you have a lot of functionality > > similar to any command shell such as history and tab-completion. The > > grub rescue> mode is more limited, with no history and no > > tab-completion. Yes, as I said, sometimes it does not work. Sometimes you just get tab characters. More usefully, if you read the 5 paragraphs in the above link starting from "What’s all this msdos stuff?" and ending at "Reading /etc/issue could be useful", then that is describing exactly where you are at now. And if you read further down, there's a heading "Booting From grub-rescue>" which describes exactly the same procedure from the grub manual that I've been suggesting you follow (that uses the grub-rescue sequence that you wrote "should work" above using the 'insmod normal' and 'normal' commands). But let's proceed step-by-step and not get ahead of ourselves. As I asked above, what does 'ls' say for each root directory?
