I'm unsure as to how to read this; are you saying because they were ZFS, the FBI couldn't read them and because of their lack of expertise, assumed they were empty?
Or that they formatted them before returning them?
Or that they formatted them before returning them?
(And by "unknown" I mean of course "anything that is not FAT* or NTFS"...)
The moral of the story is for individuals, whom should implictly fear government overreach no matter whom is in office, one has to back their shit up and make it SWAT-proof, even if that means running several TahoeLAFS boxes in countries like Switzerland, because running a server (physical or Linode) or just replicating data to a friend's server just doesn't cut it and never did.