Cunobelin wrote: ... In both cases I have been involved in the Police actually took the entire card - apparently having raw data improves the "chain of evidence" and helps avoid claims of doctoring or editing down the line ...
That's pretty much what I would expect: it's the "Best evidence rule." I'm pretty sure I've posted before that cameramen should expect this to happen and IIRC, I suggested avoiding "Readers' wives" footage on the same memory card. Somewhere on here there's advice from shootist on going to considerable lengths to avoid allegations of evidence tampering.
On reflection, I may have missed your point. I think that downloading like this might only be used for a sifting process (I originally typed screening but that's a bit ambiguous here.) ie They want to see the evidence to see if the case merits investigation. I think that if there was to be a prosecution based on the evidence on a memory card, that card would be an "exhibit." ie Part of the evidence in court. Its contents would be disclosed to the defence.