1. A disk drive comprising: a disk; a head actuated over the disk; and control circuitry operable to: sample a read signal emanating from the head to generate read samples; generate first vectors of log-likelihood ratios (LLRs) in response to the read samples, wherein each vector corresponds to a nonbinary symbol q in a set of symbols; bias the first vectors of LLRs to generate biased vectors of LLRs; and decode the biased vectors of LLRs into a data sequence, wherein the biased vectors of LLRs increase an error rate of the data sequence.