Because JW has said the defense gave BN a copy of *parts* of what was on the computer, not a copy of *everything* that was on the computer. So, if the prosecution knows which parts, they may be able to deduce what the defense's strategy is. So JW is correct in saying that JM already has everything the defense gave BN, in that JM has the entire computer that includes the parts given to BN.
In California, anything you give to an expert for review is discoverable by opposing counsel, excluding things like communications from attorney to expert and generally things prepared by the attorney for litigation. Not sure if it's the same in AZ. If it is the same, JM is entitled to a copy of the exact computer image that was provided to BN.
AZLawyer, please feel free to correct anything I've misstated