Abstract: An external diagnostic device for examining the operation of a disk drive. The device is coupled to input terminals and output terminals of the drive and operates in a selected one of several modes at a given time. In a passive mode, the drive terminals are connected to a host computer and the device monitors the presence of signals at the terminals. In a manual mode, the host computer is disconnected from the drive and the device simulates the host computer, directing control signals to the drive via appropriate input terminals. In a test mode, signals from sensors or the head in the drive are directed to the device via corresponding output terminals. Sensor and head signals are processed to determine operational parameters of the drive--such as speed, azimuth and radial alignment, sector burst, and proper reading and writing of data. Before entering the manual mode or the test mode, a step to track 00 operation is performed and a determination is made as to whether the head has been sensed at track 00.