cd to /dev/ and get hda3. To restore, cd to /dev/ and put hda3. Even though it already exists, it should write the data to the drive. It's probably not worth it to explain how Unix devices work, but suffice it to say that /dev/hda3 is a device and it's an interface to the actual partition. This is like doing Ghost, but only on a single partition, without changing its size or location.