WellPoint, one of the nation's largest health insurers, has begun notifying 75,000 members of its Empire Blue Cross and Blue Shield unit in New York that a compact disc holding their vital medical and other personal information had disappeared. The information was on an unencrypted disc that a subcontractor recently sent to Magellan Behavioral Services, a company in Avon, Conn., that specializes in monitoring and coordinating mental health and substance abuse treatments for insurance companies.