Ok, So I must be doing something wrong.
I put in my modified key. dd if=/dev/sdb of=my_key.img. done
I now put a fresh key in, reverse the process (flip dd if and of arguments). Takes forever. ANd then will not boot.
On a hunch I boot the netbook into clonezilla and make a backup of the key that way. Much faster (odd, that). Put in a fresh key, and reverse the process. WAAAY faster than dd. And it works.
I suspect I have tripped over one of the many gaps in my knowledge - can anyone enlighten me as to where I went wrong?
BTW, once I got the key backed up with clonezilla, I stuck as many keys as I could into the netbook, and did the one to multiple restore. quick. Nice trick for making a bunch of keys, though I somehow suspoect this too is common knowledge. :/