I did a dumb thing... I forgot that Ubuntu 10.04 (Lucid Lynx) switched to GRUB 2 which puts a ton of *.mod files (kernel modules) in /boot/grub. I thought they were soundtrack files put there erroneously, and I moved them. Needless to say, the next reboot was traumatic. I was presented with something I had no memory of ever seeing... a 'grub rescue>' prompt.

With the help of Fixing GRUB error: “error: unknown filesystem” however, I was able to recover...

• I discovered that GRUB rescue does not have 'cd', 'cp' or any other filesystem commands except its own variation of 'ls'.



• So first I had to find the partition with the /boot directory containing vmlinuz file and other boot image files... (approximation from memory of failed attempts, as well as blank lines for clarity, added 2014-07-10 by docsalvage)

  
  
  

  grub rescue>  ls  
  
  (hd0,4) (hd0,3) (hd0,2) (hd0,1)  
  
  
  
  grub rescue>  ls (hd0,4)/boot
  
  ... some kind of 'not found' message
  
  
  
  grub rescue>  ls (hd0,3)/boot
  
  ... some kind of 'not found' message
  
  
  
  grub rescue>  ls (hd0,2)/boot
  
  ... grub ... initrd.img-2.6.32-33-generic ... vmlinuz-2.6.32-33-generic 
  
  




• I found a /boot directory containing the vmlinuz file vmlinuz-2.6.32-33-generic on partition (hd0,2).




• Then I manually booted from the 'grub rescue>' prompt. The following commands will...

  
  
  
  
  
  • Set the root to use the /boot directory on partition (hd0,2).
  
  
  • Load kernel module linux.
  
  
  • Set that module to use the kernel image vmlinuz-2.6.32-33-generic.
  
  
  • Set initrd(initialize RAM disk) to use the image initrd.img-2.6.32-33-generic.
  
  
  • Boot Linux.
  
  



• grub rescue>  set root=(hd0,2)/boot  
  
  grub rescue>  insmod linux  
  
  grub rescue>  linux (hd0,2)/boot/vmlinuz-2.6.32-33-generic root=/dev/sda2
  
  grub rescue>  initrd (hd0,2)/boot/initrd.img-2.6.32-33-generic  
  
  grub rescue>  boot  
  
  




• This boots and crashes to the BusyBox prompt which DOES have some rudimentary filesystem commands.




• Then I moved the *.mod files back to the /boot/grub directory...

  
  
  

  busybox>  cd /boot  
  
  busybox>  mv mod/* grub
  
  busybox>  reboot
  
  




• The reboot was successful, but that was a lot of work.

Is there an easier way?