This document discusses four main types of external memory: magnetic disks, optical disks, magnetic tape, and disk drives. Magnetic disks store data on circular platters coated with magnetic material and use read/write heads to access data. Optical disks like CDs, DVDs, and Blu-rays use lasers to read and write digital information to coated discs. Magnetic tape stores data on magnetic tape housed in cartridges and uses similar reading and writing techniques as disks. Disk drives, like hard disk drives, store large amounts of data on spinning magnetic platters that are read and written to by heads positioned very close to the disk surface.