An apparatus and method are disclosed for decoding servo data recorded on a magnetic disk drive and detecting pinned layer reversals and signal errors, for example, errors due to noise. The servo data is encoded using wide-bi-phase encoding. This encoding is detected by a magneto-resistive sensor that senses the magnetization in domains passing by the sensor. The decoder includes an A/D converter for sampling the signals emitted by the sensor, to provide a sequence of the encoded data. A trellis, such as a Viterbi trellis, is employed to decode the samples generated by the converter. The trellis includes nodes representing states, connected by paths representing transitions, among the nodes. A quality value is generated for the transitions, the quality value representing the distance between each sample in the sequence output by the A/D converter and a corresponding expected sample. By applying servo data to two trellises, each corresponding to a different pinned layer magnetic orientation, and comparing the quality values produced, the correct pinned layer orientation may be selected.