How Removable Storage Works

There are several reasons why removable storage is useful:

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:

In the following sections, we will take an in-depth look at each of these technologies.

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 disks or cartridges have a few things in common:

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.

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.

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.

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.

You can read more about magnetic storage in How Hard Disks Work and How Tape Recorders Work. To learn about optical storage technology, check out the next page.

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.

That is how a normal CD works, which is great for prepackaged software, but no help at all as removable storage for your own files. That's where CD-recordable (CD-R) and CD-rewritable (CD-RW) come in.

CD-R works by replacing the aluminum layer in a normal CD with an organic dye compound. This compound is normally reflective, but when the laser focuses on a spot and heats it to a certain temperature, it "burns" the dye, causing it to darken. When you want to retrieve the data you wrote to the CD-R, the laser moves back over the disc and thinks that each burnt spot is a bump. The problem with this approach is that you can only write data to a CD-R once. After the dye has been burned in a spot, it cannot be changed back.

CD-RW fixes this problem by using phase change, which relies on a very special mixture of antimony, indium, silver and tellurium. This particular compound has an amazing property: When heated to one temperature, it crystallizes as it cools and becomes very reflective; when heated to another, higher temperature, the compound does not crystallize when it cools and so becomes dull in appearance.

CD-RW drives have three laser settings to make use of this property:

Other optical devices that deviate from the CD standard, such as DVD, employ approaches comparable to CD-R and CD-RW. An older, hybrid technology called magneto-optical (MO) is seldom used anymore. MO uses a laser to heat the surface of the media. Once the surface reaches a particular temperature, a magnetic head moves across the media, changing the polarity of the particles as needed.

A very popular type of removable storage for small devices, such as digital cameras and PDAs, is Flash memory. Flash memory is a type of solid-state technology, which basically means that there are no moving parts. Inside the chip is a grid of columns and rows, with a two-transistor cell at each intersecting point on the grid. The two transistors are separated by a thin oxide layer. One of the transistors is known as the floating gate, and the other one is the control gate. The floating gate's only link to the row, or wordline, is through the control gate. As long as this link is in place, the cell has a value of "1."

To change the cell value to a "0" requires a curious process called Fowler-Nordheim tunneling. Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually between 10 and 13 volts, is applied to the floating gate. The charge comes from the column, or bitline, enters the floating gate and drains to a ground.

This charge causes the floating-gate transistor to act like an electron gun. The excited, negatively charged electrons are pushed through and trapped on the other side of the oxide layer, which acquires a negative charge. The electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than fifty percent of the charge, it has a value of "1." If the charge passing through drops below the fifty-percent threshold, the value changes to "0."

Flash memory uses Fowler-Nordheim tunneling

to alter the placement of electrons.

The electrons in the cells of a Flash-memory chip can be returned to normal ("1") by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply this electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional electrically erasable programmable read-only memory (EEPROM) chips because instead of erasing one byte at a time, it erases a block or the entire chip.

Flash-memory storage devices such as CompactFlash or SmartMedia cards are today's most common form of electronic nonvolatile memory. CompactFlash cards were developed by Sandisk in 1994, and they are different from SmartMedia cards in two important ways: They are thicker, and they utilize a controller chip.

CompactFlash consists of a small circuit board with Flash-memory chips and a dedicated controller chip, all encased in a rugged shell that is several times thicker than a SmartMedia card. The increased thickness of the card allows for greater storage capacity.

CompactFlash sizes range from 8 MB to an incredible 4 GB. The onboard controller can increase performance, particularly on devices that have slow processors. However, the case and controller chip add size, weight and complexity to the CompactFlash card when compared to the SmartMedia card.

The solid-state floppy-disk card (SSFDC), better known as SmartMedia, was originally developed by Toshiba. SmartMedia cards are available in capacities ranging from 2 MB to 128 MB. As seen below, the card itself is quite small.

SmartMedia cards are elegant in their simplicity. A plane electrode is connected to the Flash-memory chip using bonding wires. The Flash-memory chip, plane electrode and bonding wires are embedded in a resin using a technique called over-molded thin package (OMTP). This allows everything to be integrated into a single package without the need for soldering.

SmartMedia cards are capable of fast, reliable performance while allowing you to specify the data you wish to keep. They are small, lightweight and easy to use. They are less rugged than other forms of removable solid-state storage, so you should be very careful when handling and storing them. Check out How Flash Memory Works for more information.

One of the common trends in removable storage is to make the physical package smaller while increasing the amount of data that can be stored. Take a look at these examples of each type of technology:

Magnetic

Magnetic storage is moving in two parallel directions. There are products coming out that use small cartridges with capacity measured in megabytes, and there are portable hard drives that range in the gigabytes.

Optical

A company named DataPlay has introduced a micro-optical drive. This tiny drive, about the size of a matchbox, uses tiny optical discs that are encased in a plastic shell. Each disc is capable of holding 500 MB of information. The drive actually reads both sides of the disc, meaning that the disc stores 250 MB per side.

Solid State

SmartMedia and CompactFlash cards continue to increase in capacity while maintaining their tiny size. Other solid-state memory devices, such as Sony's Memory Stick, are even smaller.

The great news for all of us is that while physical size keeps shrinking, and storage capacity keeps growing, the cost per megabyte keeps dropping! Companies like Iomega and Pockey Drives predict that you will soon be able to take your hard drive with you from one computer to the next, carrying your entire custom setup wherever you go. DataPlay's micro-optical system is a great example of a technology that will impact well beyond the desktop PC, with their drives in everything from digital cameras to MP3 players to PDAs.

For more information on removable storage, check out the links on the next page.