The document discusses various types of external memory used in computer systems, including magnetic disks, optical disks, magnetic tape, and RAID (redundant array of independent disks). Magnetic disks come in varieties such as hard disks, floppy disks, and removable hard disks. Optical disks discussed include CD-ROM, CD-R/W, DVD, and their uses, capacities, and read/write capabilities. RAID systems provide data redundancy across multiple disks for reliability and performance.

1. William Stallings Computer Organization and Architecture Chapter 5 External Memory

5. Disk Data Layout

7. Fixed and Movable Heads

27. Comparison of Disk Layouts