1. A method comprising: in a controller of a data storage device, performing: receiving data including a data block and main error correction coding (ECC) data for the data block, the data block including a first sub-block of data and a second sub-block of data; initiating an ECC operation to process the data block using the main ECC data; and in response to the ECC operation indicating uncorrectable errors in the data block, retrieving first additional ECC data that is external to the data block, the first additional ECC data corresponding to the first sub-block of data and initiating a second ECC operation to process the first sub-block of data using the first additional ECC data.