Fwd: MBR as a constant

Thorny thorntreehome at gmail.com
Sun Apr 12 15:49:19 UTC 2009


On Mon, 06 Apr 2009 11:38:12 -0500, Allen Meyers posted:

> The subject title is really not as descriptive as it should be, but here
> goes. Most windows/Linux users have windows first with the MBR as a
> constant I guess. I on the other hand have no windows but 8.10 is my
> constant (I thought) on sda. Debian is sdb on external. My question is
> when I upgrade to 9.04 on sda1 does MBR as a matter of course load with
> it into my internal HD? Like when I loaded Debian the question was asked
> as 2 drives were detected where to put grub MBR or? I know this sounds
> confused, but I just wanted it spelled out because the OS on my HD is
> not a constant and I am windowless
> 
> 
Allen. I am a bit concerned because there has been no response from you to
Brian's explanation. Concerned because you've already had some trouble
understanding GRUB and have expressed a reluctance to ask questions here
and worried that you may have tried the upgrade you mentioned and might be
having trouble booting your system. It's been six days since you asked the
question and you generally have follow-up questions or a mention of
success. Maybe my concern is misplaced, it could be that you understand
things fine now and just don't have any follow-up questions or you may be
busy with your real life. If so, just ignore this post.

As Brian explained, only the first part of GRUB (512 bytes) is written to
The MBR (of your first enumerated disk, in your case sda) and it has the
information that sends GRUB to the partition where the next stages (the
rest of GRUB) reside. You will remember that /boot/grub is a directory on
your Ubuntu install (if that is the constant you mean)(Start using
partitions in your descriptions, we can't be sure what you mean when you
say something is your constant on sda because sda has partitions, do you
mean sda1, that's what I'm assuming for the rest of my explanation?)[It
could be different if you have some other operating system that was
installed first on your computer (I think I remember you mentioning Mepis
once in the past). Chances are very good that your MBR portion of GRUB
points to the partition of your first installed operating system.] Since
the MBR portion of GRUB already points there, you need make no changes to
that MBR. When your system boots, it will go to (sda1)/boot/grub and,
after finding the next stage it needs, look for menu.lst (GRUB's
configuration file) and present you with the menu. For the sake of
simplicity (over simplify), the MBR tells GRUB where to find the menu and
as long as it remains in the same place on the same partition the MBR can
be considered constant.

Now going logically forward, if you make changes to the partition that
holds /boot/grub (sda1), for example, upgrading to a newer version, the
GRUB menu.lst will have to change to reflect the new stanza necessary to
boot that new operating system (it will be a different kernel and initrd
image.) The MBR portion of GRUB on sda (no partition here, it's the MBR
area of the physical drive) can remain the same, it still points to
(sda1)/boot/grub. When you do the upgrade, it should, after installing the
new kernel, update GRUB (this means update /boot/grub/menu.lst) with the
new information for booting the upgraded operating system and leave the
stanzas for your other multi-boot operating systems choices as they were,
as long as they were in the correct location in the file. Note that I am
assuming you didn't do something like format sda1 and do a clean install.
Anything that messes with the /boot/grub directory on that sda1 partition
can effect what GRUB finds there when the MBR sends it there. In that case
(clean install), you would likely only be able to boot to the newly
installed operating system until you make some corrections. It is for that
case that I previously told you to save a backup of your menu.lst in a
safe place, if you trashed the old one you still have the backup to refer
to for the correct stanzas for booting your other operating systems as
they should still be on their same partitions.

Since in the past you've expressed to me a difficulty understanding these
bootloader things because you've only come recently to the open source
software world, I may be trying to explain something that you have already
grasp well, I just don't want to take a chance of leaving you out on a
limb if you require help. I do understand how it becomes increasingly
difficult to understand and adopt new concepts as one becomes older, it's
happening to me too. You deserve our patient help because you have reached
77, not likely you got that far without "paying your dues" along the way,
by one method or another.





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