MARKING UNRELIABLE SYMBOLS IN A HARD DISK DRIVE READ BACK SIGNAL

A system for detecting errors in a channel comprises a decoder that generates first data when a number of symbols in error in a first detected sequence from the channel is less than or equal to a predetermined number, and that generates a failure indication when the number is greater than the predetermined number. A controller causes the decoder to receive a second detected sequence from the channel when the decoder asserts the failure indication and identifies corresponding symbols of the first and second detected sequences that differ.