=====Grub2=====

How to boot from USB with grub2:

<code>
grub> ls
(hd0) (hd0,msdos5) (hd1) (hd1,msdos0)
</code>

This will show you the available devices

<code>
grub> linux (hd1,msdos1)/install/vmlinuz root=/dev/sdb1
grub> initrd (hd1,msdos1)/install/initrd.gz
grub> boot
</code>

====Install - Repair====

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/