Method and apparatus for alternatively reading or writing data to a servo surface of a hard disk drive

A circuit for enabling the writing of servo data by a servo transducer to a dedicated servo surface of a hard disk drive during the manufacture of the drive while enabling only the reading of servo data at subsequent times that the drive is mated to a computer. The circuit receives power on selected terminals of a connector and includes a level detector that provides a first control signal in response to a power supply voltage level that is below a level selected to exceed voltage levels available from the power supply of a user computer and a second control signal in response to a power supply voltage above the selected level. The second control signal is transmitted to a switch assembly that responds by providing a direct connection between the servo transducer and data terminals of the connector. The first control signal is transmitted to switches by means of which an amplifier, having inputs connected to the transducer and outputs connected to the data terminals, is supplied with electrical power. Servo patterns are written at the time of manufacture of the hard disk drive by operating the circuit from a power supply that provides a voltage level that exceeds the selected level and are read by operating the circuit from the computer power supply.