Hi all,

I had an erratic behavior from my system lately (debian "testing") after upgrading all the packages, I had a kernel panic message at boot and thus was stuck.
After fiddling a bit, I did a new fresh debian install that detected my old system and by some luck was able to boot my old system without the panic message once I added the intird option in the menu.lst of the new system next to the old kernel entry (and then giving the initrd image as parameter of course).
Now, I'd like to reinstall the old system's grub in the MBR (I added the initrd line in it) but have no idea how. Updare-grub does not seem to be doing the job (at least it doesn't say so in the man page).
Can anybody tell me what to do, please?

If you know what partition the system is on it helps. You can actually use grub to help you find that.
So boot up into Linux and get to the grub tool with the command: grub
At the grub prompt use the following comands....
grub > find /sbin/init
That should get the available Linux partitions. For example, on my box .....
grub> find /sbin/init
(hd0,0)
(hd0,4)
(hd1,1)

Now, you use the partition for whichever distro you want to boot the grub menu from ....
grub> root (hd1,1)
Filesystem type is ext2fs, partition type 0x83

Now, you can tell grub to use the MBR with the command: setup (hd0)

grub> setup (hd0)
Checking if "/boot/grub/stage1" exists... yes
Checking if "/boot/grub/stage2" exists... yes
Checking if "/boot/grub/e2fs_stage1_5" exists... yes
Running "embed /boot/grub/e2fs_stage1_5 (hd0)"... 15 sectors are embedded.
succeeded
Running "install /boot/grub/stage1 d (hd0) (hd0)1+15 p (hd1,1)/boot/grub/stage2 /boot/
grub/grub.conf"... succeeded
Done.

Then exit the grub with this command:
grub> quit