The expectation of privacy ends where? The man was no longer in possession of his data, in essence, he handed it to someone else.
This isn't about whether the guy skates on the child porn charge or not. Best Buy is not a state actor, so anything they do has no affect on the charges. However, is it ethical for Best Buy to rummage through someone's hard drive? I'm no techie, so I don't know. The question I guess is whether they need to go through the hard drive to do their job and just came by this by accident or do they routinely look through peoples' hard drives for a few laughs and maybe to mine personal data.
And if he had it on video tape or a magazine in the back of his car and some mechanic found it? It is still his even if it isn't in his possession.