I am in IT. If it has a different hard drive it is very easy to tell. Secondly the FBI has leaked that they think they can recover “some” of the emails, so I don’t think it is a different hard drive. Even with a military grade wipe there is still a good chance some if not the vast majority of it will be recoverable. It is extremely hard to perfectly wipe a drive, I dare say almost impossible. Also, supposedly there is a back up server.
As the emails were forwarded from gov’t servers, The FBI will recover them from those servers, is what I understand.
There was someone stripping the “Classified” markings and pages from the emails, copying them and sending them to the bathroom server from non-gov’t email account. Used to be called “sneaker net” back in the day.