1. CD-ROM has become a popular medium for distributing software, multimedia data, and other electronic published information.

2. Capacity of CD-ROM: ～500 MB. Disks are cheap to mass produce and also drives.

3. CD-ROM: much longer seek time (250m/sec), lower rotation speed (400 rpm), leading to high latency and lower data-transfer rate (about 150 KB/sec). Drives spins at 8/12< × audio CD spin speed (standard) is available.

4. Recently, a new optical format, digit video disk (DVD) has become standard. These disks hold between 4.7 and 17 GB data.

5. WORM (write-once, read many) disks are popular for archival storage of data since they have a high capacity (about 500 MB), longer life time than HD, and can be removed from drive - good for audit trail (hard to tamper).