A serious data breach at the Securities and Exchange Commission transferred personal data about current and former employees into the computer system of another federal agency, a letter sent by the SEC to staff reveals.The July 8 letter, obtained by The Hill, is from Thomas Bayer, the SEC’s chief information officer and senior agency official on privacy. It warned that personal employee data had been discovered on the networks of another, unnamed federal agency.It said a former SEC employee “inadvertently and unknowingly” downloaded the names, birthdates and Social Security numbers of employees onto a thumb drive, and then transferred them...