IF your / root partition and /usr partition are not at the same disk partition ??
How to rescue grub under Debian lirnux ?
1. boot from an Debian CD
2. start rescue mode , after setting network and clock , ALT+F2 enter a Terminal shell
若在 安裝網路之前，則 fdisk 不動作
3. fdisk -l , find the root partition, such as /dev/sda2
要了解 partition 內容可以 再 mount 之後進入看其內容來判斷之
4. # mkdir myroot ,
# mount /dev/sda2 /myroot,
5. # mount -o bind /proc /myroot/proc
proc 再 chroot 之後可以 看 partition
#mount -o bind /dev /myroot/dev
#mount -o bind /sys /myroot/dev
6. # chroot /myroot
change root to /myroot
7. # fdisk -l , find the /usr partition , such as /dev/sda7
#mount /dev/sda7 /usr , since grub-install is at /usr/sbin.
8. #update-grub , a new grub.cfg is as /boot/grub/
copy origin grub.cfg to /boot/grub
#cp /usr/lib/grub/grub.cfg /boot/grub/.
9. #grub-install /dev/sda
10 . reboot.
