Why would Lanza smash his hard drive? He was on a murder/suicide mission, whatever was found on his hard drive would be totally irrelevant. However, if others were involved in the shooting, they would want to destroy evidence.
Because he wanted to hide or destroy something on it. Or IF he had a conspirator or encourager, perhaps they directed him to do so.
Maybe he was hiding his kiddie porn collection, who knows.
The point is, they won't know until they know and it would be procedurally stupid to let any information out at all until they are sure there was no one behind Lanza, pushing him over his own psychotic cliff.