Abstract: In a magnetic disk drive of the type using data erase portions as address marks, an address mark detection system has an address mark detecting circuit and a read data masking circuit. The detecting circuit, monitoring a read data signal, becomes active on detecting a data erase portion whose duration is shorter than a predetermined period of time for address mark detection. The detecting circuit is cleared on detecting a data input. The masking circuit, receiving the read data signal, masks any suitable number of read data pulses input immediately after the detecting circuit has been cleared. As a result, even when noise is introduced in the data erase portion, the system prevents the resulting false read data pulses from being output.