Because the term 'disc' or 'disc' refers to several different memory formats, 'removable disk' designates those which can be removed and replaced without opening the computer's case. There are a number of different types of removable discs, such as; A floppy disc. Originally an 8 inch disc, this format has shrunk to 3 1/2 inches. Many new computers do not have floppy disc drives, because the format can only hold 1.4 megabytes per disc. There have been several 3 1/2 inch optical disc drives created, which are similar to CD-R/W drives, in that the disc is written to and read by a laser. These drives can be mounted in place of a floppy drive, and have capacities of 20 megabytes, or more. CD-ROM -R/W The compact disc, a 5 inch optical Read Only Memory format, is generally thought of as an audio format, but was adapted for use by computers shortly after its introduction. Many computers can now boot from a CD, and most software is sold on CD. The Read/Write CD drive is now over ten years old, and was introduced as a way to store and transfer data. Drives that can write to a CD are called 'burners', but can only write to special CD's, called CD-R/W or Read/Write. DVD-ROM - R/W. With the introduction of the Digital Video Disc, the amount of data that could be stored on a 5 inch optical disk increased dramatically. The DVD format has gone through the same evolution as the CD format, in that a burner type drive was introduced some years after the ROM drive. However, DVD discs will not work in CD drives, although most DVD drives can read CD's. Generally, a computer will refer to CD and DVD drives under the general heading of CD-ROM drives. The most common use of 'removable disc' is the 3 1/2 inch floppy.