A method of operating a memory device includes writing initial data to non-volatile memory cells of a non-volatile memory cell array, generating a difference value based on a difference between first data related to the initial data written to the non-volatile memory cells and second data related to the initial data written to the non-volatile memory cells, comparing the difference value with a reference value, and generating and transmitting a status signal indicating that the initial data has been successfully written to a controller when the difference value is less than the reference value. The data may be randomized by the controller or the memory device.