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persantine iv As with Seagate, WD was already selling a "hybrid" laptop drive (also called a solid state hybrid drive or just SSHD), which combines a small amount of NAND flash (SSD) with a traditional spinning disk. Like the new WD Black Dual Drive, WD's SSHD uses a 5400rpm spinning disk. The difference between the two drives is that the SSHD stores all data on the spinning disk and uses the SSD portion of its storage as cache to serve up frequently used data, such as the OS during boot ups and for apps when they're loading.