Well, you just said the problem yourelf. You opened your hard drive. Unless you did this in a 'clean room', that has almost guaranteed to have detroyed the destroyed it.
One fleck of dust could have taken out quite a few tracks as it gets picked up by the head and dragged all over the place. You could probably get away with opening it in a non clean room if the drive was empty and you did a surface scan after closing it and reallocated all the newly-bad sectors, however even that would be very risky.