Ok guys, little help needed if you don't mind

I flashed my Xbox to the latest Freeboot the other week, no problems.
A mate just gave me his box to update as well. I've just dumped the NAND through Flash360, and booted Xell to get the CPU key. All good. However, when I try and create the image in Bestpig, it just stops at 38% and gives me the message "kv.bin was not decrypted properly, CPU key does not match the dump".
Not sure what's going on there exactly, unless the guy who JTAG'ed the box initially played funny buggers with a donor nand or something.
Anyway, how do I go about getting this working? I do have my NAND dump from the other week, can I somehow use that to fix up this other one?
Thanks!
Not sure if you got an answer on this but i had exactly the same problem, first off i had copied the cpu key around the wrong way (5 then 3 instead of 3 then 5 ) not sure if that makes a difference. I also just this minute tried my 2nd nand dump and now i have my freeboot image ready to write, i have xbr on now so i dont know if i can just write straight over it ?