which grub to edit?

David G. Miller dave at davenjudy.org
Thu Jun 28 16:51:38 UTC 2007

"Mikkel L. Ellertson" <mikkel at infinity-ltd.com> wrote:

> Thufir wrote:
>> > pardon if I posted this before.  I was working on grub last night.
>> > Win2k and Fedora 7 are on sda (hda) while FC3 is on sdb (hdb).  The
>> > MBR on sda is being used, I believe.
>> > 
>> > Which grub do I edit to get all OS's booting?  I tried a few
>> > variations, mucking with the one in /boot/grub/... from FC3, but
>> > wasn't able to hit on the correct combination.
>> > 
>> > 
> Chances are you will want to edit the one from F7. It depends on
> what the last run of grub-install put on the MBR of the boot drive.
> Mikkel
Chance's are that Mikkel is correct but it's whichever disk has an 
"active partition" (bootable) and that will be found first given the 
boot order set in the BIOS.  Chances are that this is sda.  Confirm your 
BIOS boot order says to try sda first and that the partition containing 
/boot on that drive is active.  If you wanted to, you could have grub on 
sdb controlling booting by adding the F7 and W2K partitions to it's 
grub.conf and running grub-install under FC3 with the correct incantations.

The BIOS checks boot devices in the order specified through the BIOS 
setup.  When it finds the first device that meets the criteria given 
through setup, the BIOS reads, loads and then executes some (fairly 
trivial) code on the same sector that holds the partition table.  This 
code reads the partition table and looks for an active partition.  The 
MBR of the active partition is loaded and executed.  This is where grub 
gets installed so this will hopefully be grub which then finds your 
grub.conf and does whatever you have specified.

As an example, the boot device on my server (CentOS 4.5) is /md3 (RAID 1 
mirror) that consists of /dev/hde and /dev/hdg but I've told the BIOS to 
use those drives instead of /dev/hda.


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