I have a multiboot WinXP/Ubuntu Hard Drive but something has gone wrong and multiboot stopped working. So, my access to Ubuntu installation is lost on the HDD. How do I restore the multiboot function to return to Ubuntu? Thanks a lot
You have probably overwritten your bootsector. Try to use the Supergrubdisk. If that for some reason does not work, you can try the guide from the german ubuntu users wiki:
Start from a Ubuntu installation CD (or any other linux that runs from CD like knoppix). Then mount your Ubuntu root partition from the HDD to some folder, for instance /mnt. If you have a separate /usr, /boot or /etc partition, please mount it in the corresponding subfolder of the root parition (so if root is mounted at /mnt, then boot should be at /mnt/boot and so on). When finished, you can simply "sudo grub-install --root-directory=/mnt /dev/sda" (of course only if your hdd is sda).
If the last step doesn't work, because your installation is too outdated, you should still be able to use the chroot-method also given at ubuntuusers.de: Mount the root and boot partition as written above and bind the /dev directory so that you also have it available within the mounted root partition (assuming the root partition is mounted at /mnt):
Same for /sys and /proc
Now you can chroot
And you should be ready to run "grub-install /dev/sda". It's also a good idea to run "update-grub" afterwards.