did a large post preupgrade yum update, Grub now just says "GRUB"

[marko]

Can someone remind me what the problem is with grub when all
it says to the screen at reboot is:

GRUB


and then hangs there? I did a huge post F8->F9 preupgrade yum update,
then on the next reboot attempt this is happening. I have a
copy of systemrescuecd on a usb key I can use to fix the grub with
but I'm not sure what's wrong. For some reason on this machine
I can't boot Fedora cd/dvd's anymore to do linux rescue, it just says
it can't find a fedora partition on the disk and ejects it asking for
another.

[stoat]

Originally Posted by marko

Can someone remind me what the problem is with grub when all
it says to the screen at reboot is:

GRUB


and then hangs there? I did a huge post F8->F9 preupgrade yum update,
then on the next reboot attempt this is happening.

Hello marko,

It means (to me, anyway) that the stages did not all run. Probably, stage1 in the MBR was loaded into memory and executed but could not find the subsequent stage which is usually stage1.5 when GRUB is installed in the MBR. Stage1 is so tiny that it has to have the sector address of the next stage embedded in it at the time it is created (offset 44h-47h). If that address changes for any reason, then stage1 cannot find the next stage and stops after printing the word "GRUB" followed by a space on the screen. There is usually a blinking underline following the space, and the system is hung there requiring a reset. Now I don't know the underlying explanation for why this is happening after this F9 update, but you are not alone (for what it's worth). In that bugzilla link below, Will Woods discovered a difference in the stage1 code in the MBR before and after the update. The change is to the bytes that identify the boot drive and the sector address of the next stage. He speculates about the root cause of it all. Anyway, re-installing GRUB by the usual methods seems to make everyone happy again.

https://bugzilla.redhat.com/show_bug.cgi?id=450143
http://forum.fedoraforum.org/forum/s...d.php?t=191893
http://forum.fedoraforum.org/forum/s...d.php?t=188465
http://forum.fedoraforum.org/forum/s...d.php?t=191140
http://forum.fedoraforum.org/forum/s...d.php?t=189290
http://forum.fedoraforum.org/forum/s...d.php?t=180663

[marko]

What's so odd is that all I did was a yum update, there was a kernel
update but I would think that a kernel update would only
edit the grub.conf and not have to mess with the grub in the MBR.
The grub didn't need to move or be altered AFAIK

[stoat]

Well, according to Will Woods' observation, the actual stage does not necessarily become relocated. What he observed is a change in the sector address for the next stage in stage1 in the MBR. Personally, I would not have expected any yum update involving any package to affect the GRUB stages in any way. Like you, I also would expect a kernel update to only edit the grub.conf.

[marko]

FIXED, I had to run this due to my weird disk geometry
grub on hda with Win XP and RedHawk Linux,
hdb for Fedora 9 X86-64 (grub stage 1 on hda
and grub stage 2 and grub.conf on hdb )

> root (hd0)
> setup (hd0) (hd1,0)
> quit

My critical thing was, don't chroot the disk as offered by
the rescue "Continue" option, pick "Skip" and run
grub shell via absolute path. chroot otherwise interfered with
my grub shell mounting the disk .