Because flash memory now provides an economical solution for various portable devices and embedded systems, an NAND flash-based storage system has replaced the hard disk drive in many applications. Recently, the implementation of database systems using an NAND flash-based storage system has become an important research topic. In particular, the external sorting is an important operation in database systems. With the very distinctive characteristics of flash memory, the typical external sorting system that adopts a clustered sorting process can result in performance degradation and reduce the reliability of flash memory. In this article, we will propose an unclustered sorting method that considers the unique characteristics of flash memory, and we then propose a decision rule to exploit the advantages of both clustered and unclustered sorting. The decision rule can separate records according to their record length, sort them appropriately by the clustered and unclustered sorting, and merge the sorted results. The experimental results show that the proposed method can improve performance in an NAND flash-based storage system (i.e., solid-state drive).