A storage device, including a nonvolatile memory device including a plurality of memory cells forming a user area and a reserved area; and a controller configured to perform a write operation on at least one write unit included in the plurality of memory cells, and to perform an erase operation on at least one erase unit included in the plurality of memory cells, wherein the controller includes an internal buffer, wherein the controller is configured to: perform a backup operation by writing first data stored in the internal buffer in a backup erase unit included in the reserved area, and after performing the backup operation adjust a buffering unit of the internal buffer to correspond to a cell type of the backup erase unit.