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?
I read it as his drives were formatted as ZFS and the FBI couldn't read them because of their incompetence (probably only thinking to try and read them with Windows as FAT32 or NTFS.
Judging by how Windows, when presented with an unknown FS, acts as if the disk were unformatted and immediately offers to format it, my guess would be both...
(And by "unknown" I mean of course "anything that is not FAT* or NTFS"...)
Johnny clubfingers 5-0 clicks Ok, and destroys "evidence" for being a total idiot... I take it he/she was hungover during that 2 hours of computer forensics in the academy. For crooks, this is good news; for stopping potential future victims, this isn't good, and misapplied to innocent/MPAA/RIAA enforcement, it's destructive and lowers LE credibility.
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.
Or that they formatted them before returning them?