How Removable Storage Works ...

Removable storage has been around almost as long as the computer itself. Early removable storage was based on magnetic tape like that used by an audio cassette. Before that, some computers even used paper punch cards to store information!

We've come a long way since the days of punch cards. New removable storage devices can store hundreds of megabytes (and even gigabytes) of data on a single disk, cassette, card or cartridge.

Portable Memory

There are several reasons why removable storage is useful:
Commercial software
Making back-up copies of important information
Transporting data between two computers
Storing software and information that you don't need to access constantly
Copying information to give to someone else
Securing information that you don't want anyone else to access

Modern removable storage devices offer an incredible number of options, with storage capacities ranging from the 1.44 megabytes (MB) of a standard floppy to the upwards of 20-gigabyte (GB) capacity of some portable drives. All of these devices fall into one of three categories:

Magnetic storage
Optical storage
Solid-state storage

Magnetic Storage

The most common and enduring form of removable-storage technology is magnetic storage. For example, 1.44-MB floppy-disk drives using 3.5-inch diskettes have been around for about 15 years, and they are still found on almost every computer sold today. In most cases, removable magnetic storage uses a drive, which is a mechanical device that connects to the computer. You insert the media, which is the part that actually stores the information, into the drive.

Just like a hard drive, the media used in removable magnetic-storage devices is coated with iron oxide. This oxide is a ferromagnetic material, meaning that if you expose it to a magnetic field it is permanently magnetized. The media is typically called a disk or a cartridge. The drive uses a motor to rotate the media at a high speed, and it accesses (reads) the stored information using small devices called heads.

Each head has a tiny electromagnet, which consists of an iron core wrapped with wire. The electromagnet applies a magnetic flux to the oxide on the media, and the oxide permanently "remembers" the flux it sees. During writing, the data signal is sent through the coil of wire to create a magnetic field in the core. At the gap, the magnetic flux forms a fringe pattern. This pattern bridges the gap, and the flux magnetizes the oxide on the media. When the data is read by the drive, the read head pulls a varying magnetic field across the gap, creating a varying magnetic field in the core and therefore a signal in the coil. This signal is then sent to the computer as binary data.

Magnetic: Direct Access

Magnetic disks or cartridges have a few things in common:
They use a thin plastic or metal base material coated with iron oxide.
They can record information instantly.
They can be erased and reused many times.
They are reasonably inexpensive and easy to use.

If you have ever used an audio cassette, you know that it has one big disadvantage -- it is a sequential device. The tape has a beginning and an end, and to move the tape to later song you have to use the fast forward and rewind buttons to find the start of the song. This is because the tape heads are stationary.

A disk or cartridge, like a cassette tape, is made from a thin piece of plastic coated with magnetic material on both sides. However, it is shaped like a disk rather than a long, thin ribbon. The tracks are arranged in concentric rings so the software can jump from "file 1" to "file 19" without having to fast forward through files 2 through 18. The disk or cartridge spins like a record and the heads move to the correct track, providing what is known as direct-access storage. Some removable devices actually have a platter of magnetic disks, similar to the set-up in a hard drive. Tape is still used for some long-term storage, such as backing up a server's hard drive, in which quick access to the data is not essential.

The read/write heads ("writing" is saving new information to the storage media) do not touch the media when the heads are traveling between tracks. There is normally some type of mechanism that you can set to protect a disk or cartridge from being written to. For example, electronic optics check for the presence of an opening in the lower corner of a 3.5-inch diskette (or a notch in the side of a 5.25-inch diskette) to see if the user wants to prevent data from being written to it.

Magnetic: Zip

Over the years, magnetic technology has improved greatly. Because of the immense popularity and low cost of floppy disks, higher-capacity removable storage has not been able to completely replace the floppy drive. But there are a number of alternatives that have become very popular in their own right. One such example is the Zip from Iomega.

The main thing that separates a Zip disk from a floppy disk is the magnetic coating used. On a Zip disk, the coating is of a much higher quality. The higher-quality coating means that the read/write head on a Zip disk can be significantly smaller than on a floppy disk (by a factor of 10 or so).

The smaller head, in conjunction with a head-positioning mechanism that is similar to the one used in a hard disk, means that a Zip drive can pack thousands of tracks per inch on the disk surface. Zip drives also use a variable number of sectors per track to make the best use of disk space. All of these features combine to create a floppy disk that holds a huge amount of data -- up to 750 MB at the moment.

Magnetic: Cartridges

Another method of using magnetic technology for removable storage is essentially taking a hard disk and putting it in a self-contained case. One of the more successful products using this method is the Iomega Jaz. Each Jaz cartridge is basically a hard disk, with several platters, contained in a hard, plastic case. The cartridge contains neither the heads nor the motor for spinning the disk; both of these items are in the drive unit

Magnetic: Portable Drives

Completely external, portable hard drives are quickly becoming popular, due in a great part to USB technology. These units, like the ones inside a typical PC, have the drive mechanism and the media all in one sealed case. The drive connects to the PC via USB cable and, after the driver software is installed the first time, is automatically listed by Windows as an available drive.

Another type of portable hard drive is called a microdrive. These tiny hard drives are built into PCMCIA cards that can be plugged into any device with a PCMCIA slot, such as a laptop computer.

Optical Storage

The optical storage device that most of us are familiar with is the compact disc (CD). A CD can store huge amounts of digital information (783 MB) on a very small surface that is incredibly inexpensive to manufacture. The design that makes this possible is a simple one: The CD surface is a mirror covered with billions of tiny bumps that are arranged in a long, tightly wound spiral. The CD player reads the bumps with a precise laser and interprets the information as bits of data.

The spiral of bumps on a CD starts in the center. CD tracks are so small that they have to be measured in microns (millionths of a meter). The CD track is approximately 0.5 microns wide, with 1.6 microns separating one track from the next. The elongated bumps are each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers (billionths of a meter) high.

Most of the mass of a CD is an injection-molded piece of clear polycarbonate plastic that is about 1.2 millimeters thick. During manufacturing, this plastic is impressed with the microscopic bumps that make up the long, spiral track. A thin, reflective aluminum layer is then coated on the top of the disc, covering the bumps. The tricky part of CD technology is reading all the tiny bumps correctly, in the right order and at the right speed. To do all of this, the CD player has to be exceptionally precise when it focuses the laser on the track of bumps.

When you play a CD, the laser beam passes through the CD's polycarbonate layer, reflects off the aluminum layer and hits an optoelectronic device that detects changes in light. The bumps reflect light differently than the flat parts of the aluminum layer, which are called lands. The optoelectronic sensor detects these changes in reflectivity, and the electronics in the CD-player drive interpret the changes as data bits.