A switch assembly is provided for a tape drive adapted to receive a tape cartridge. The assembly comprises a housing, first and second switches fixedly mounted on the housing, and first and second plungers mounted for reciprocal movement on the housing to actuate the first and second switches, respectively. The housing includes three orthogonal reference surfaces, all of which engage the tape drive to locate the assembly in proper relationship to the tape drive. The plungers are responsive to the tape cartridge to actuate the first switch when the tape cartridge is properly mounted on the drive and to actuate the second switch when the tape cartridge is not write-protected. The assembly may include a light source and a light-responsive switch which, together with a reflector in the tape cartridge, cooperate to detect position-indicating tape apertures.