Follow the timeline...."Only a few hours after the New York office of the FBI took possession of the Weiner laptop, on September 26, 2016, the FBI computer expert discovered it contained more than 140,000 emails involving Hillary Clinton."

"according to the inspector general, 39 high-ranking FBI agents knew of it, along with the New York office and people in the New York U.S. Attorney’s office. The New York FBI informed them all during a secure video-conference on September 28 "

So SDNY knew of this on September 28, 2016. Remember that's Prett's office, so he knew this on that day.