Contact potential difference (CPD) experiments were conducted by the vibrating probe (Kelvin) method on humidity-exposed hard disk drive surfaces. The measurements were made at various locations along the circumference of a wear track caused by the sliding contact with a spherical silicon pin, operated in a pin-on-disk configuration. The load on the pin, rotational speed of the disk, and the humidity were controlled. The CPD signals varied along the wear track and the magnitude of the CPD change increased with the load from 29 to 98 mN at a fixed relative humidity. The CPD signals also increased linearly with relative humidity ranging from 30% to 70% as the normal load was fixed at 29 and 49 mN. The CPD signal appears to saturate at a load of 98 mN where the relative humidity is 50% and higher.