Ubuntu – Need to re-install grub2 to /boot pbr (not mbr). Use live cd and the grub-install

dual-bootgrub2installation

I have dual boot system(Windows XP and Ubuntu 12.04) that was running fine until an auto-update of grub2 this past week-end.

I had been using the Windows boot loader to choose the O/S, which allowed me to retain the original Microsoft MBR on the hard drive. The setup had grub2 in a /boot partition (/dev/sda3) visible to the system BIOS in the first 137mb. Grub2 had been installed to the PBR in that partition. I had modified the Windows boot.ini file to point to a binary file copied from the /boot pbr, per a procedure whose URL escapes me at the moment, but is well known to dual-booters I believe.

I can run the latest Ubuntu 12.04 live CD and get access to all partitions on the hard drive. I can run fdisl -l and blkid and see all of the particulars of the various partitions. What I am uncertain of is the exact incantation to use to cause grub-install to do only what I want to do to fix up my boot partition, assuming grub-install is indeed the way to go.

Incidentally, I did look at the boot-repair tool and did not see it offering me the option of installing grub to other than the hard drive mbr. My boot partition is /dev/sda3 and the only option boot-repair appeared to permit for grub re-installation was to /dev/sda, rather than /dev/sda3. I take that to mean boot-repair will only install to the MBR, and not the PBR of /dev/sda3, though the menu labels are bit cryptic and I am guessing at meanings here.

I'm thinking the grub-install script is the way to go, but my first attempt produced an error message that suggested it could not find a /boot directory. I suspect the live CD session and its separate file system may be causing confusion about that. I assume there is a specific syntax I should be using to get around that, but am just about at the limit of my know-how.

I hesitate to tinker with syntax or script options and risk irreparably damaging my installation. I would be most appreciative if an expert coder would suggest a grub-install command line that will re-install grub2 on /dev/sda3 when executed from a terminal command line in an Ubuntu 12.04 live CD session. For educational purposes, it would also be nice to have an explanation of the rationale for the specific syntax.

Best Answer

Mount your / partition in /mnt and your /boot partition in /mnt/boot, then chroot into the system and run dpkg-reconfigure grub-pc and choose the install location from the menu.

sudo -s
mount /dev/sda2 /mnt
mount /dev/sda3 /mnt/boot
for f in dev sys proc dev/pts ; do mount --bind /$f /mnt/$f ; done
chroot /mnt
dpkg-reconfigure grub-pc
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