This is a computer guy. Nobody would be surprised if he had spare hardware lying around from as far back as the 90s. If he just quick-formatted a bunch of spare drives and tossed them on a shelf years ago any forensics software would find what was on them before he formatted them. Considering that he's been collecting pictures since he was 14 I think it would be hard not to get him for possession if he's a hardware pack-rat.
That's a good point. I would probably forget about old drives as well - I thought they found the pictures on an active drive but realise now that this was a baseless assumption.
