Quotes from Elliot B. Koffman

Most personal computers use two types of disk drives as their secondary storage devices-hard drives and optical drives. Hard disks are attached to their disk drives and are coated with a magnetic material. Each data bit is a magnetized spot on the disk, and the spots are arranged in concentric circles called tracks. The disk drive read/write head accesses data by moving across the spinning disk to the correct track and then sensing the spots as they move by.

Flash drives such as the one pictured in Fig.1.6 use flash memory packaged in small plastic cases about three inches long that can be plugged into any of a computer's USB (Universal Serial Bus) ports. Unlike hard drives and optical drives that must spin their disks for access to data, flash drives have no moving parts and all data transfer is by electronic signal only. In flash memory, bits are represented as electrons trapped in microscopic chambers of silicon dioxide.