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Removable storage devices
When computers were first invented, their main purpose was to process data and produce useful information. However the usefulness of information depended (and still does) on how effectively that information could be communicated. Since the invention of the computer, at an age where the Internet and such networks were decades away in the future, removable storage systems played a key role in the evolution of the computer into the indispensable device it is today.

Portable information storage devices were necessary for many reasons including the sale and distribution of commercial software, for 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 someone else and securing information that you don’t want anyone else to access being a few.

The magnetic tape drive which performed a function very similar to that of a cassette recorder was a big leap forward as far as removable storage was concerned. However, 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 and solid-state storage.

Magnetic storage
Last week we discussed floppy disks, which fall into this category. Just like a hard drive, the media used in all removable magnetic-storage devices are 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.

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

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, which means that the read/write head on a Zip disk can be significantly smaller than on a floppy disk.

The smaller head and a head-positioning mechanism that is similar to those used in a hard disks, enables Zip drives to pack thousands of tracks per inch on the disk surface. All of these features combine to create a floppy disk that can hold up to 250 MB!
Another method of using magnetic technology for removable storage is 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 heads and the motor for spinning the disk are in the drive unit.

Completely external, portable hard drives are becoming popular, due in great part to USB technology. These units, like the ones inside a PC, have the drive mechanism and the media in one sealed case. The drive connects to the PC via USB cable and is automatically listed by the computer 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), which we discussed a few weeks ago. A CD can store huge amounts of digital information on a very small surface that is incredibly inexpensive to manufacture. The CD player reads the bumps with a precise laser and interprets the information as bits of data, thus it is categorized as optical storage devices together with DVDs.

Solid state storage
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 means that there are no moving parts.

Here is a brief description of how Flash memory works. Inside the chip, there 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 tunnelling.

Tunnelling 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 storage devices such as ‘CompactFlash’ or ‘SmartMedia’ cards are today’s most common form of electronic nonvolatile memory. CompactFlash cards 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. 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. They are used often in mobile phones.

The future of removable storage devices
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. 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. The good news is that while physical size keeps shrinking and storage capacity keeps growing, the cost per megabyte keeps dropping! - Source: PCHardwareGuide.com

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