Cannot update grub with parameters on live USB

You'll need to do a bit of remounting and remapping.

After booting the liveCD, we'll mount the Ubuntu partition to /mnt with:

sudo mount /dev/sd*# /mnt

where /dev/sd*# is updated to your Ubuntu partition.

If you have a separate /boot partition, we'll need to mount it in /mnt/boot as well:

sudo mount /dev/sd*# /mnt/boot

where /dev/sd*# is updated to your /boot partition.

This should mount it with enough access to get the canonical path if needed, but we'll likely not need this.

To finish up, just update grub with the correct root and target:

sudo grub-install --root-directory=/mnt /dev/sdX

where /dev/sdX is the disk(not the partition) where we want GRUB.