I've only put together a computer twice and I know that you could not distribute porn that is "put in there by the original owner" without manually going into the torrent network via your torrent client since he can't use the OS that was installed in the hard drive, the installation of the torrent client if it was a fresh install from the top down would not know where to find the file to distribute, and you have to download the torrent information manually by looking for it in the internet.
Did I get anything wrong with this?
Mostly.
Assuming it's a used drive with pre-existing OS and user config, and assuming he was able to boot it or make it bootable on a different system (doable), he'd still have to have pre-existing user credentials or the account would have to be blank password, then he'd have to go out of his way to fire up an allegedly pre-configured torrent client that already had existing torrent data of the CP in question.
But... the one big problem there is that the very first thing LEO would do is verify the data particulars on the hard drive (after they CoC'd all the evidence and imaged it with the image getting it's own CoC), first and foremost being the file timestamps to match up with their investigative log client side of his activity. They KNOW when it was first downloaded to the machine. If that date is after he acquired the hard drive, he's fucked.
Actually he's fucked regardless because he's still guilty of distributing whether he was aware of it or not (but realistically, they got him on the data download as well because obviously it's a bullshit story).
If it was a secondary drive for storage then your scenario is accurate and he's even more retarded because in order for him to unknowingly share such material he would, as you said, have to link that directory up for torrenting (or move the data where he is sharing it) AND reshare the actual damn torrent (a scenario even dumber than the system drive scenario), which legally still has him on the hook.
I'm all for innocent until proven guilty but I know what I know, and there's a lot of other people hanging around here that have private/commercial/enterprise experience with PC/IT and his story is fucktardedly unrealistic.
Regardless, he'll get his day in court. "Good luck. You'll need it!" -Rackets