A method of operating a memory system includes; storing data in a buffer region of the nonvolatile memory, later issuing a migration request directed to the data stored in the buffer region and executing a migration operation to move the data from buffer region to a main region of the nonvolatile memory device. Upon completion of the migration operation, marking a migration operation completion time, and after an initial verify shift (IVS) time has elapsed following the migration operation completion time, updating a mapping table associated with the data in view of the executed migration operation.