no monta el raiz: Mounting /dev/hd on /root failed: No such device

[Daniel Jimenez]

hola gente,
con la última actualización automática del kernel se me ha ido todo al 
carajo.
Yo tengo instalado Dapper.
Os pego las salidas de los distintos arranques del grub por si me podéis 
echar una mano:

si arranco con lo primero que me ofrece el grub --> Ubuntu, kernel 
2.6.15-28-686
____________________________________________________
    Booting 'Ubuntu, kernel 2.6.15-28-686'
root (hd0,1)
 Filesystem type is ext2fs, partition type 0x83
kernel /boot/vmlinuz-2.6.15-28-686 root=dev/sda4 to quier splash
 [Linuz-bzImage, setup=0x1c00 .....
initrd /boot/initrd.img-2.6.15-28-686
  [Linuz-initrd @ ....
savedefault
boot
Uncompressing Linux... Ok, booting the kernel.
mount: Mounting /dev/sda4 on /root failed: No such device
mount: Mounting /root/dev on /dev/.static/dev failed: No such file or 
directory
... (no monta nada que empiece por /root, claro)
Target filesystem doesn't have /sbin/init
-------------------------------------------------------

si arranco con -->Ubuntu, kernel 2.6.15-28-386
_____________________________________________________
Booting 'Ubuntu, kernel 2.6.15-28-386'
root (hd0,1)
 Filesystem type is ext2fs, partition type 0x83
kernel /boot/vmlinuz-2.6.15-28-386 root=dev/sda4 to quier splash
 [Linuz-bzImage, setup=0x1c00 .....
initrd /boot/initrd.img-2.6.15-28-386
  [Linuz-initrd @ ....
savedefault
boot
Uncompressing Linux... Ok, booting the kernel.
mount: Mounting /dev/sda4 on /root failed: No such device
mount: Mounting /root/dev on /dev/.static/dev failed: No such file or 
directory
... (no monta nada que empiece por /root, claro)
Target filesystem doesn't have /sbin/init

BusyBox v1.01 (Debian 1:1.01-4ubuntu3) Built-in shell (ash)
Enter 'help' for a list of built-in commands.
#_
-----------------------------------------------
si arranco con  -->Ubuntu, kernel 2.6.15-28-386 (recovery mode)
_______________________________________________
(un monton de mensajes con formato [xxxxxxxx:xxxxxx] mensajes...
Begin: Running /scripts/local-premount...
[17179580.732000] Attempting manual resume
Done.
mount: Mounting /dev/sda4 on /root failed: No such device
Begin: Running /scripts/local-bottom ...
Done.
Done.
mount: Mounting /root/dev on /dev/.static/dev failed: No such file or 
directory
Done.
mount: Mounting /sys on /root/sys failed: No such file or directory
mount: Mounting /proc on /root/proc failed: No such file or directory
Target filesystem doesn't have /sbin/init

BusyBox v1.01 (Debian 1:1.01-4ubuntu3) Built-in shell (ash)
Enter 'help' for a list of built-in commands.
#_
----------------------------------------------

y así ando, más despistao que un pulpo en un garaje.
He buscado este error en la red pero no he encontrado nada de mucha ayuda.
He probado a repara el grub con SuperGrub:
http://supergrub.forjamari.linex.org/
http://users.bigpond.net.au/hermanzone/SuperGrubDiskPage.html
pero nada de nada.

Toda ayuda será muy bienvenida.
Gracias.
Dani.