Abstract: The invention disclosed reduces unrecoverable read errors from a magnetic tape resulting from the apparent lack of a cyclic check character (CCC), sometimes referred to as a cyclic redundancy check character, or a longitudinal check character (LCC), sometimes referred to as a longitudinal redundancy check character. The detector provides a variable read window to synchronize receipt of the CCC and/or LCC with their associated data blocks even though the CCC and/or LCC was not written in the appropriate time frame called for by the write specification of the tape in question.