I changed the UUID label in Grub in line : kernel /boot/vmlinuz-2.6.20-15-generic root=/UUID=.... by : kernel /boot/vmlinuz-2.6.20-15-generic root=/dev/sda1 ro quiet splash and now it works.
So apparently there are some confusion with UUID labelling
I changed the UUID label in Grub in line : 2.6.20- 15-generic root=/UUID=.... 2.6.20- 15-generic root=/dev/sda1 ro quiet splash
kernel /boot/vmlinuz-
by :
kernel /boot/vmlinuz-
and now it works.
So apparently there are some confusion with UUID labelling