The quality of substrate that exists between the optical head and the media sensitive layer is known to critically influence the drive performance. First, we report here the degradation of the servo and read signals by the parallelism of the two surfaces of the plastic substrate. It is conclusive that the peak to peak variation of plastic parallelism in the radial direction over one revolution can cause tracking instability and data cross talk. Consequently, the seek accuracy and bit timing accuracy in the optical drive are degraded. Second, we report that the variation of birefringence over one revolution results in the variation of light feedback into the laser cavity. This increases the laser noise, and thus degrades the read performance, especially when the laser output power is in the lower range.