Patrick F. Cadigan and James C. Fosberg have been elected to the board of directors of Media Systems Technology, a Santa Ana manufacturer of automatic diskette duplication products. With their election, the MST board now has five members. Last year, the six-member board was reduced by half when three directors resigned. Cadigan is chairman of Linear Instruments Corp., Irvine. Fosberg is executive vice president, chief operating officer and director of Cyber Systems, Anaheim.