There are privacy rights in the workplace, but particularly in the private workplace they're limited. Contexts where an invasion of privacy is more likely to be found? Video surveillance in restrooms and changing rooms, or the physical search of an employee. The extent of worker privacy rights varies by state. For example, some, but not all states have found a right to privacy in items locked in an assigned locker, secured by the employee's lock. But even in states more inclined to extend rights, that can vary by policy and based upon notice to the employees. But we're getting pretty far afield from the monitoring of workplace computer usage.

If the question here is workplace privacy, I don't think the employee gets very far. If it's the employer's unreasonable refusal to return the hard drive after inspecting its contents and finding nothing improper, the employee should get his hard drive back.