Fwd: MBR as a constant

[Allen Meyers]

On Sun, Apr 12, 2009 at 10:49 AM, Thorny <thorntreehome at gmail.com> wrote:
> 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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Thorny and list
I was in fact still having some concerns and I guess I did not address
them being rather reluctant to belabor a point.
But out of the blue comes Thorny's post and suddenly it was as if a
light went on and I thank him for that detailed and user friendly
response. So for now I understand the MBR, and grub and it
relationship to my OS and partitioning. You all are indeed a great
resource and I know I will have some questions unrelated to MBR. Right
now I am goggling like the dickens on how to get to my media
partition. So you may get that concern as well, but I owe it to myself
and you all to learn what I can first.
Happy Easter and thanks again

Allen