A doctor who works at Brigham and Women's and Faulkner hospitals lost an external hard drive in June, and the computer device may have contained medical information for 638 patients, the hospitals said. The Brigham has sent letters to those patients describing the problem. Information related to inpatient hospital stays from July 10, 2009, to Jan. 28, 2011, may have been on the device, including patient names, medical record numbers, dates of admission, medications, and information about diagnosis and treatment. The device did not contain Social Security numbers, insurance numbers, or other financial account information. The doctor lost the hard drive June 21 while traveling in Mexico. But hospital officials said the physician had previously taken steps to delete patient information from the device, so the chances of information remaining on it are low. The Brigham would not identify the physician.