Ubuntu Grub2 修復Grub2方法

Recover Grub 2 via LiveCD

• First, grab a copy of the latest Ubuntu LiveCD and boot it.

• Open a terminal and type
• ----------------------------------------quick recovery begin-----------------------
• $ sudo fdisk -l
• $ sudo mount /dev/sda1 /mnt ##注意使用 你的 root分區所在的位置，也就是 / 所在的分區，可使用 Ubuntu自帶的磁盤實用工具查看
• $ sudo mount /dev/sda2 /mnt/boot ##若是有專用的啓動分區的話 掛載它
• $ sudo mount --bind /dev /mnt/dev #這3個 比較必要，參數不用改
  $ sudo mount --bind /proc /mnt/proc
  $ sudo mount --bind /sys /mnt/sys
• $ sudo chroot /mnt
• # update-grub #已是root了
• # grub-install --recheck /dev/sda #通常一塊硬盤的話，不用改了
• # reboot #重啓 就能夠了
• --------------------------------quick end--------------------------

$ sudo fdisk -l

• Now, you need to remember which device listed is your linux distribution, for reference, /dev/sda1 will be used. Now we need to mount the filesystem to /mnt

$ sudo mount /dev/sda1 /mnt

• If you have /boot on a separate partition, that need's to be mounted aswell. For reference, /dev/sda2 will be used.

$ sudo mount /dev/sda2 /mnt/boot

• If you have /var on a separate partition, that need's to be mounted aswell. For reference, /dev/sda3 will be used.

$ sudo mount /dev/sda3 /mnt/var Make sure you don't mix these up, pay attention to the output of FDISK

• Now mount the rest of your devices and some other things needed in the chroot

$ sudo mount --bind /dev /mnt/dev
$ sudo mount --bind /proc /mnt/proc
$ sudo mount --bind /sys /mnt/sys

• Now chroot into your system

$ sudo chroot /mnt

$ update-grub

You should be chroot'd into your system as root, you can now run commands as root, without the need for sudo.

• Now you need to edit the /etc/default/grub file to fit your system

$ nano /etc/default/grub

• When that is done you need to run update-grub to create the configuration file. If you have a separate /boot partition you need to mount it first!

$ update-grub

• To install GRUB 2 to the MBR, next you need to run grub-install /dev/sda

$ grub-install /dev/sda

• If you encounter any errors, try grub-install --recheck /dev/sda

$ grub-install --recheck /dev/sda

• Press Ctrl+D to exit out of the chroot.
• Once you exit back to your regular console, undo all the mounting, first the /dev and others

$ sudo umount /mnt/dev
$ sudo umount /mnt/sys
$ sudo umount /mnt/proc

• Now you can unmount the root system. (But if you have a separate boot partition which you mounted earlier, you have to unmount this first, or you will get a "device busy" error message.)

$ sudo umount /mnt

• And you should be free to restart your system right into GRUB 2 and then into your system installation.

If you had alternate OS entries, update-grub might say "Cannot find list of partitions!". Ignore it and continue - once you can boot into your linux installation, do so and then rerun update-grub and grub-install /dev/sda as root.

Restore GRUB2 - Recovering from a Windows XP / Vista / 7 Reinstallation

Fire up a terminal from the Live CD for Ubuntu 10.04.
$ sudo fdisk -l (Note the partition number on which Linux resides)
$ sudo mount /dev/sdaX /mnt (Replace X with the partition number housing Linux)
$ sudo grub-install --root-directory=/mnt/ /dev/sda
$ sudo update-grub
$ sudo reboot


Credits to
http://mundogeek.net/archivos/2009/12/08/recuperar-grub-2/ for the enlightening post & 
http://www.webupd8.org/2009/12/how-to-recover-grub2-linux.html for the awesome translation.

重點推薦 Ubuntu Grub2 徹底手冊：https://help.ubuntu.com/community/Grub2#Reinstalling%20GRUB%202