A hard disk drive is disclosed including a voice coil motor and a magnetic sensor communicating with a control circuit. A magnetic sensor reading is created and used to stop access operations in response to the reading of an external magnetic field that may be strong enough to disrupt a slider accessing a rotating disk surface. Also disclosed are a main flex circuit and head stack assembly including the magnetic sensor for use in a hard disk drive. Control circuits and processors are disclosed for receiving the magnetic reading to stop access operations in hard disk drives. The hard disk drive may be a Perpendicular Magnetic Recording (PMR) hard disk drive.