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Re: Trying to use Grub Command Line to boot from CD

From: Jesse Thompson
Subject: Re: Trying to use Grub Command Line to boot from CD
Date: Fri, 7 Jun 2013 07:43:27 -0700

Thank you Jordan. The drives appear to be some kind of a SCSI Raid5 hardware array (this is among the reasons I'm not just pulling the drives to glean their data via another machine), but I think the CDROM is still PATA and no other PATA devices should be important to the data recovery process (there's like a 12/24gb DAT drive, but that's not getting used ever again. :P).

So would the prefix variable be important if I'm trying to boot from media (CDROM) different from the drive Grub2 was loaded from? Or does it mean all or part of Grub2 cli will begin to malfunction until that prefix gets repaired? Or does it mean the CDROM rescue environment I boot into may not be able to see the SCSI drive object unless I do something with the prefix before booting?

Once I get back to this machine this afternoon, I'm thinking along the lines of:
grub> ls
(confirm the CDROM isn't visible but hd is)
grub> insmod pata
grub> ls
(confirm the CDROM is visible now)
grub> ls (cd??)
(explore around and find the linux image and initrd to set, and then set them and boot)

I'll report back how that all goes. :)

- - Jesse

On Fri, Jun 7, 2013 at 1:55 AM, Jordan Uggla <address@hidden> wrote:
On Thu, Jun 6, 2013 at 3:06 PM, Jesse Thompson <address@hidden> wrote:
> Hello!
> I would like some help trying to boot from CD using Grub command line.
> Trying to recover data from an ancient Compaq Proliant server which lacks a
> way to set boot order in bios.
> Only apparent thing I know how to access is Grub command line.
> "ls" does not display the CD as a device, and I'm not sure if that means CD
> drive itself is borked or inaccessable, or if I should insmod something to
> make it visible.

For whatever reason, BIOSs only present the CDROM drive to bootloaders
if the machine was booted from CD, so there is probably no way to
access the CDROM drive through your BIOSs interfaces, but luckily grub
does have its own native drivers as well.

To load grub's native drivers you need to insmod the appropriate
modules, which of course depends on what type of interface this drive
uses. The modules available  are "pata", "ahci" (SATA), "uhci" (one of
the specifications for USB 1.0), "ohci" (the other), "ehci" (USB 2.0),
and "usbms" (For mass storage devices connected via ohci,uhci, or

Loading "ahci" or "pata" will disable grub's access to drives via BIOS
interfaces (having two sets of software trying to access the hardware,
grub and the BIOS, at the same time could be dangerous) and so you'll
have to change grub's $prefix variable to point to the new device name
so that grub can find its modules. There is a new command recently
added (or soon to be added) to grub trunk which automates this (fixing
$prefix when loading native drivers). And a final caveat, you cannot
use chainloading when using grub's native drivers, you must load the
kernel from the CDROM directly.

Jordan Uggla (Jordan_U on

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