You can try to backup your partition onto the external. You need to figure out which disk is the external withthen runCode:sudo fdisk -lsubstitute the X.Code:sudo dd if=/dev/sda2 of=/dev/sdX
That was a typo in my earlier post. It should be /dev/sda. That dd command will take a long time, and it won't give you any feedback about how far along it is. Just backup your mbr and run the parted command from my earlier post.