NAND-flash-based SSDs (solid-state disks) are recently used in embedded computers to replace power-hungry disks. This paper presents a hybrid approach to large SSDs, which combines MLC flash and SLC flash. The idea is to complement the drawbacks of the two kinds of NAND flash with each other's advantages. The technical issues of the design of a hybrid SSD pertain to data placement and wear leveling over heterogeneous NAND flash. Our experimental results show that, by adding a 256 MB SLC flash to a 20 GB MLC-flash array, the hybrid SSD improves over a conventional SSD by 4.85 times in terms of average response. The average throughput and energy consumption are improved by 17 % and 14%, respectively. The hybrid SSD is only 2% more expensive than a purely MLC-flash-based SSD, for which the extra cost is limited and very rewarded.