This is not for the experts, as there was very little to fix. It's for the Linux newbies who go "aagh!!!" when they've installed the security update that SUSE's update applet told them to, and then found they couldn't boot any more. And it's so simple that it will save them a lot of surfing and reading posts too technical to understand. Only two things are required: that the "/boot" directory is on the same partition as the "/" (root) directory, and that there is another bootable Linux partition in the grub menu. Because trying to work from a rescue CD didn't work too well.
One night, the automatic update applet in openSUSE 11.3 announced five critical security updates, one of which involved the kernel and had to be done manually because a Wacom USB driver's dependencies could not be satisfied. I let the other four run automatically and then, as proposed by the applet, opened YaST and, from the options offered, chose to uninstall the Wacom USB driver as I don't have a Wacom tablet anyway. And now the kernel was to be upgraded, from 2.6.34-12 to 220.127.116.11-0 to be exact. This worried me a little as kernel upgrades have never gone well for me, the one or two times I tried (although, those times, I tried to "bake" a kernel following instructions, and always ended up with something unbootable). So when, after the update, my SUSE partition wouldn't boot any more, I wasn't surprised.
Having a separate "/home" partition and Zenwalk, Mint and even Windows to fall back on, I wasn't too upset. Still, it was annoying. So I surfed again. Apparently it was normal for kernel updates, especially in SUSE versions, to break support for USB drivers, leave a messed-up kernel that had to be re-downloaded, and corrupt either the menu list of grub, the default boot loader, or the file called initrd. In my grub menu list, I could see the entries for both kernel versions. Choosing the newer one gave an error 21, telling me that the partition was not found (although booting into Zenwalk showed me that it was still there, undamaged and mountable) while choosing the older one gave error 15: file not found (because that kernel had been removed, duh). Since grub still showed a menu, the most likely problem was a bad initrd (a disk image file needed for booting). The command for creating a new initrd is "mkinitrd", run presumably as root and presumably from the root directory of the afflicted Linux installation.
After a bit of messing around, booting up from the install DVD and choosing
"system rescue" and then trying to mount the partition and use chroot to run
mkinitrd from the right place, and then some websurfing because it didn't work,
I found another solution, booted into Zenwalk Linux (another partition on the
same disk), opened a terminal, made myself superuser with "su" and password, and
umount /dev/sda[n] mkinitrd -d /dev/sda[n]
I rebooted, chose the new kernel and again got error 21. This time I noticed
that what couldn't be found was (hda1,[n]) which in grubspeak means the nth
partition on the second disk. But the laptop only has one! Then I
restarted and, on getting the grub menu, typed in what I wrote down when surfing
Linux forums, and should have used in the first place:
c(this gives access to the grub command line)
find /boot/grub/menu.lst(I knew perfectly well where it's located, but I wanted to see if grub knew too, and it gave me the answer in grubspeak: (hd0,10) meaning the eleventh partition on the first harddisk)
root (hd0,10)(this is grub's chroot)
kernel /boot/vmlinuz(tells grub which kernel to use, ie. vmlinuz in /boot of the partition I chose)
initrd /boot/initrd(ditto for initrd)
boot(speaks for itself)
So maybe there was nothing wrong with the initrd file(s) and the only problem had been the erroneous disk number in /boot/grub/menu.lst. In which case I could just have booted into SUSE from the grub command line and corrected menu.lst straight away. That's how simple fixing a boot problem can be!
Postscript: within a week of writing this, I had the second kernel update to
18.104.22.168-0.2, to fix the aforementioned USB driver problems. Same thing:
menu.lst wasn't updated properly and had to be edited. There was nothing wrong
with initrd. This has happened with every subsequent kernel update, so after
every update, I edit menu.lst before restarting.