I can't wait to find out what was on their hard drive that freaked the "MOM" out - after your child is murdered, why would anything freak you out after that?
According to an earlier story, the hard drive contained four-years' worth of the family's digital pictures. So, maybe because of their previous experiences with the DHS and law enforcement, they hid the hard drive to keep it safe. Maybe they worried that, if the police get possession of it, that could very well be the last they'll ever see of the undamaged hard drive and/or the precious photographs it contains.
That's pretty much the only innocent explanation I can come up with. Of course, thinking of not-so-innocent explanations is a lot easier.