Storage devices and methods of operating storage devices

A storage device includes a NAND flash memory device, an auxiliary memory device and a storage controller to control the NAND flash memory device and the auxiliary memory device. The storage controller includes a processor, an error correction code (ECC) engine and a memory interface. The processor executes a flash translation layer (FTL) loaded onto an on-chip memory. The ECC engine generates first parity bits for user data to be stored in a target page of the NAND flash memory device based on error attribute of a target memory region associated with the target page, and selectively generates additional parity bits for the user data under control of the processor. The memory interface transmits the user data and the first parity bits to the NAND flash memory device, and selectively transmits the additional parity bits to the auxiliary memory device.