Professor Orin Kerr has wrote about this exact case extensively, and provides a good insight into all legal aspects. I think it is well worth a read, especially the part about the 'forgone conclusion'.
Right. Well that changes things.
There is some evidence of the contents.
I wonder if there are hash values for those 20,000 CEM files and if they are still on freenet. If so LE could aquire, via the hashes, from the network, and prove, without opening the drives, what the contents are. At least it becomes a strong circumstantial case. If you were relying on a single file you could claim the odds of hash collision etc, but after a few hundred I think you are well past that.
There is a pattern of behaviour here.
*edit - aquire not require..