Western Digital, or WD, develops HDDs and SSDs. The WD assortment includes both internal and external hard drives. Their external HDDs are available in a 2.5-inch size for on the go and 3.5-inch size for your desk. You can use them as soon as you take them out of the box. Internal hard drives are also available in 3.5 inches for use in your NAS. SSDs are faster than these hard drives. Some SSDs, such as M.2 NVMe SSDs, are up to 20 times faster.