You make interesting points, but I think you are over-complicating it. But then I tend to see things in pretty stark terms.
IMO, provided the storage media (and I don't care if it's a fully equipped working computer, or simply a peripheral component) came into your possession by ethical means - i.e. obtained in good faith, the former owner's contents are their concern. If they have not sanitised, that's on them.
The device, and its data, are now yours (state statute aside).