Not being an IT person, I'm not sure how much can be retrieved once hard drives are formatted. I know simply delete is not truly delete, but what about formatting? If she did format her server drive, then what's left? Plus, she can always say that she told her people to delete non-essential data and her flying monkeys inadvertantly formatted by mistake.
Bottom line: She was using a non-secure server for high level classified comms in her position of SoS. That is really what the inquiries should be focused on. What she did was illegal. Someone posted the State Department regs and she was in violation which is a criminal offense.