What made her confident that she would get away with it? Perhaps she figured that if Sandy Berger could pilfer the National Archives and escape with probation, she could surely hide a few years worth of emails without any repercussions.

For those who've forgotten that jaw-dropping story:

According to reports from the Inspector General of the National Archives and the staff of the House of Representatives' Government Operations Committee, Mr. Berger, while acting as former President Clinton's designated representative to the commission investigating the attacks of September 11, 2001, illegally took confidential documents from the Archives on more than one occasion. He folded documents in his clothes, snuck them out of the Archives building, and stashed them under a construction trailer nearby until he could return, retrieve them, and later cut them up. After he was caught, he lied to the investigators and tried to shift blame to Archive employees. Contrary to his initial denials and later excuses, Berger clearly intended from the outset to remove sensitive material from the Archives. He used the pretext of making and receiving private phone calls to get time alone with confidential material, although rules governing access dictated that someone from the Archives staff must be present. He took bathroom breaks every half-hour to provide further opportunity to remove and conceal documents... What could have been important enough for Berger to take the risks he did?

Or perhaps she was thinking farther back to the time when paperwork subpoenaed during the Whitewater investigation mysteriously disappeared for two years:

After nearly two years of searches and subpoenas, the White House said this evening that it had unexpectedly discovered copies of missing documents from Hillary Rodham Clinton's law firm that describe her work for a failing savings and loan association in the 1980's. Federal and Congressional investigators have issued subpoenas for the documents since 1994, and the White House has said it did not have them. The originals disappeared from the Rose Law Firm, in which Mrs. Clinton was a partner, shortly before Mr. Clinton took office.

Where were the records finally found? In the third-floor private residence at the White House.

From that same story:

Investigators have also been trying to determine whether Mrs. Clinton sought to block Justice Department lawyers from examining files in Mr. Foster's office in the days after his death. White House aides searched the office and brought files to the Clintons' private residence before allowing investigators to look at them.

There was also last year's delay in releasing papers that the public was entitled to see:

A trove of Clinton White House records long processed for release remains hidden from public view at the Clinton Presidential Library in Little Rock — even though the legal basis initially used to withhold them expired more than a year ago. The papers contain confidential advice given to or sought by President Bill Clinton, including communications with then-first lady Hillary Clinton, and records about people considered for appointments to federal office.

One thing Hillary Clinton may have been hiding with her private email address was noted by John Cook a couple years ago when Sidney Blumenthal's email got hacked:

Blumenthal, a fierce Clinton partisan in the 1990s, was the orchestrator of a subterranean smear campaign against Obama during the Democratic primary and was specifically spiked by the White House as a potential staffer for Clinton when she became Secretary of State. And he was sending notes to Clinton at a private, non-governmental email address. Did Obama know Clinton was consulting with the guy who tried to kneecap him? Blumenthal has long been a fomenter of dark whisper campaigns against Clinton enemies. He was famously accused in the 1990s by his former friend Christopher Hitchens of spreading the lie that Monica Lewinsky was a mere "stalker" who shouldn't be taken seriously. More recently, as Gawker reported, he was busy compiling scurrilous opposition research dossiers about Barack Obama's purported ties to unwholesome black preachers that ended up in Fox News chief Roger Ailes' inbox. That sort of behavior is, presumably, why the Obama White House put the kibosh on Clinton's attempt to hire Blumenthal at the State Department in 2009. But that apparently didn't stop Clinton from using him in an unofficial capacity.

There's no telling what other official or semi-official business Clinton conducted through the private email account that she seems to have begun using just before taking her cabinet job. The emails she has turned over were all chosen by Clinton and her aides, giving her an opportunity to hide whatever she wants hidden. But the episode already confirms what attentive observers have long known: If the Clintons return to the White House, we can expect more suspicious secrets, stonewalling, and opaqueness, much as we've seen in the past. Voters have been given ample warning.