The only germane cases are ones with close parallels to the Clinton case involving mishandling of classified information without being able to prove either damage or intent.
Jem is absolutely right and you are talking out your ass. One reason there are no previous cases is no other cabinet official has ever been this reckless.
Comey did not cite even one such failed prosecution. There is no need to prove damage. There was no damage in the case of the Naval Reservist. There was no damage in the case of the sailor who emailed his parents a picture of the reactor. There was no damage in the Petraeus case. Yet all were prosecuted.
Intent is not necessary here, but there was plentiful evidence of it. Reckless disregard is equivalent to intent. As Trey Gowdy drew out today, there were a series of untruthful exculpatory statements from Clinton, which is evidence of intent and knowledge of wrongdoing. Hillary went to elaborate lengths to set up this system and took great care to keep it secret, again indicative of intent. She was warned repeatedly it was not appropriate.
At the end of the day, the question is a simple one. Are high officials held to a lesser standard than ordinary people? Comey's position clearly is that they are.