How to boot from USB with grub2:
grub> ls (hd0) (hd0,msdos5) (hd1) (hd1,msdos0)
This will show you the available devices
grub> linux (hd1,msdos1)/install/vmlinuz root=/dev/sdb1 grub> initrd (hd1,msdos1)/install/initrd.gz grub> boot
Boot your system with a live-linux such as systemrescuecd:
http://www.system-rescue-cd.org/Download/
Prepare install on the running system:
mkdir /mnt/root mount /dev/sda2 /mnt/root/ #sda2 contains the root-filesystem. You can mount as well a lvm-based partition! mount -o bind /proc /mnt/root/proc/ mount -o bind /dev /mnt/root/dev mount -o bind /sys /mnt/root/sys chroot /mnt/root /bin/bash export PATH=$PATH:/bin export PATH=$PATH:/usr export PATH=$PATH:/usr/sbin
Note:“/mnt/root/” is the change root directory, where the root-filesystem has to be mounted!
To install run
grub-install /dev/sda
or
grub2-install update-grub
inside chroot, where “/dev/sda” is the boot-disc.
Check also on:
http://www.system-rescue-cd.org/disk-partitioning/Repairing-a-damaged-Grub/