If the new partition is really unallocated, try deleting the unallocated partition and then create the mirror from the existing disk, selecting the empty space when prompted to.

I'm going to suggest that he was hoping for full agent behavior -- that is, load the key on his main host and then be able to forward it along with his chain of ssh hops as he goes. My understanding (and experimentation today) is that this is not possible -- mosh does not forward agent information on to child ssh sessions because it doesn't know how to.