A method of operating a memory system includes storing data received from an external device in a buffer memory of the memory system, programming the data stored in the buffer memory to a first storage area of a nonvolatile memory of the memory system in response to a mode of the memory system being in a guarantee mode and to a second storage area of the nonvolatile memory in response to the mode of the memory system being in other than the guarantee mode, and programming the data stored in the first storage area to the second storage area during an idle time.