A data encoding method includes storing K input data symbols; assigning the symbols to respective symbol locations in a notional square array, having n rows and n columns of locations, to define a plurality of k-symbol words in respective rows; encoding the words by encoding rows and columns of the array dependent on a product code having identical row and column codes, each being a reversible error-correction code of dimension k and length n=2n′, thereby to define a codeword, having n2 code symbols corresponding to respective locations of the array, of a quarter product code defined by CQ={X−XT−(X−XT)F: X∈C} where X is an n by n-symbol matrix defining a codeword of the product code, XT is the transpose matrix of X, and (X−XT)F is a reflection of matrix (X−XT) in the anti-diagonal thereof.