A check sum calculation on data coded with a Reed-Solomon error correcting code is performed by applying a byte based polynomial remaindering process to data bytes. The polynomial is X2nullXnull2nullnull, over GF (28), where null is the primitive element GF (28) used to define redundancy coding for individual data groups. The roots of the polynomial used in the polynomial remaindering process differ from the roots of a generator polynomial of the Reed-Solomon error correcting code. The polynomial remaindering process is performed with a sub function mask containing the same mask function as used in defining redundancy coding for a data group or groups. The data group or groups are redundance coded using a Reed-Solomon code over GF (28)