Is this machine dedicated to Mandrake, or are you dual booting with another OS? Is the harddrive hda (master on the primary IDE channel), or something else?
Have you tried to look at the partitions as Mandrake set them up, using <fdisk /dev/hdx>.
If you have an installation CD from another linux distro, have you tried using it to set up the partitions to see if it will let you?
Would you be willing to reinstall? I understand that fixing the problem is your first choice , but it may not be the easiest one.