ePMR – Enhanced Perpendicular Magnetic Recording. A Western Digital disk drive data recording technology which applies an electrical current to the main pole of the write head throughout the write operation, a bias current. This current generates an additional magnetic field which creates a preferred path for the magnetisation flip of media bits. This, in turn, produces a more consistent write signal, significantly reducing jitter.

The need for such a bias current is that disk recording heads can provide an inconsistent magnetic field to bits because their write currents can be distorted – so-called “jitter”. This effect makes bit value signal recognition more difficult and worsens as bits decrease in size and are placed closer together.