Ubuntu option disappear from grub 11.10 after update on a dual partition

Question

After today's update Nov. 22, 2011 Ubuntu option disappear from grub 11.10 on my dual partition.

some of fdisk -l:

Device Boot      System
/dev/sda1   *    HPFS/NTFS/exFAT
/dev/sda2        Extended
/dev/sda5        Linux
/dev/sda6        Linux swap / Solaris

Device Boot    ID System
/dev/sdb1   *  c  W95 FAT32 (LBA)

I also ran this:

 grub-probe -t device /boot/grub

to see where the grub was got this error:

 grub-probe: error: cannot find a device for /boot/grub (is /dev mounted?).

did this too:

 sudo update-grub

got the same error: /usr/sbin/grub-probe: error: cannot find a device for / (is /dev mounted?).

I also tried to boot repair Gui but it only shows MBR option and no grub. when I turn the computer on I get:

No such file:
grub rescue>

Any help would be much appreciated.

Edit: I also mounted the linux partition:

  sudo mount /dev/sda5 /mnt
  mount: /dev/sda5 already mounted or /mnt busy
  mount: according to mtab, /dev/sda5 is already mounted on /mnt

and ran the above commands and got the same errors as above.

Also did as tohuwawohu instructed:

umount /mnt
sudo fsck -p /dev/sda5
fsck from util-linux 2.19.1
/dev/sda5: clean,

Thanks again. Any Help would be awesome.

Answer 1

Maybe the boot repair tool will help (see also this answer). You can install it inside a live system using:

sudo add-apt-repository ppa:yannubuntu/boot-repair

sudo apt-get update && sudo apt-get install -y boot-repair && boot-repair

Here's some information how to proceed in grub rescue mode.