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• What is USB Flash Driver

USB flash drives are NAND-type flash memory data storage devices integrated with a USB (universal serial bus) interface. They are typically small, lightweight, removable and rewritable. As of April 2007, memory capacities for USB Flash Drives currently are sold from 32 megabytes up to 64 gigabytes. Capacity is limited only by current flash memory densities, although cost per megabyte may increase rapidly at higher capacities due to the expensive components. (USB Memory card readers are also available, whereby rather than being built-in, the memory is a removable Flash memory card housed in what is otherwise a regular USB flash drive, as described below.)

USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk. They are more compact, faster, hold more data, and are more reliable due to their lack of moving parts, and more durable design. These types of drives use the USB mass storage standard, supported natively by modern operating systems such as Windows, Mac OS X, Linux, and Unix.

A flash drive consists of a small printed circuit board typically in a plastic or metal casing, making the drive sturdy enough to be carried about in a pocket, as a key fob, or on a lanyard. Only the USB connector protrudes from this protection, and is usually covered by a removable cap. Most flash drives use a standard type-A USB connection allowing them to be connected directly to a port on a personal computer.

To access the data stored in a flash drive, the drive must be connected to a computer, either by plugging it into a USB host controller built into the computer, or into a USB hub. Flash drives are active only when plugged into a USB connection and draw all necessary power from the supply provided by that connection. However, some flash drives, especially high-speed drives, may require more power than the limited amount provided by a bus-powered USB hub, such as those built into some computer keyboards or monitors. These drives will not work unless plugged directly into a host controller (i.e., the ports found on the computer itself) or a self-powered hub.


• Technology

Flash memory is actually a combination of a number of older technologies, the low cost, low power consumption and small size being made possible by recent advances in microprocessor technology. The memory storage is based on earlier EPROM technology. The first EPROMS had very limited capacity, were very slow for both reading and writing, required complex high-voltage drive circuitry, and could only be re-written by first exposing them to short-wavelength ultraviolet light, which would erase the entire contents.

A later variant, the EEPROM could be erased electrically without the need for UV light, but with the original versions, it was still a total erasure. (These were widely used for storing the BIOS firmware in PCs).

Later EEPROMS were developed where the erasure region was broken up into smaller "fields" that could be erased individually without affecting the others. Altering the contents of a particular memory location involved first copying the entire field into an off-chip buffer memory, erasing the field, and then re-writing the data back into the same field, making the necessary alteration to the relevant memory location while doing so. This required considerable computer support, and PC-Based EEPROM flash memory systems often carried their own dedicated microprocessor system. Flash drives are more or less a miniaturized version of this.

The development of high speed serial data interfaces such as USB for the first time made serially accessed storage memory systems viable, and the simultaneous development of small, high speed, low power microprocessor systems allowed this to be incorporated into extremely compact systems. Serial access also greatly reduced the number of electrical connections required to the memory chips, which has allowed the successful manufacture of multi-Gigabyte capacities. (Every external electrical connection is a potential source of manufacturing failure, and with traditional manufacturing, a point is rapidly reached where the successful yield approaches zero).

Modern flash memory systems are accessed very much like hard disk drives, where the controller system has full control over where information is actually stored. The actual EEPROM writing and erasure processes are however, still very similar to the earlier systems described above.

Many low-cost MP3 Players simply add extra software to a standard flash memory control microprocessor so it can also serve as a music playback decoder. Most of these can also be used as a conventional flash drive.



• Strengths and weaknesses

Strengths

Flash drives are nearly impervious to the scratches and dust that were problematic for previous forms of portable storage, such as compact discs and floppy disks, and their durable solid-state design means they often survive casual abuse. This makes them ideal for transporting personal data or work files from one location to another, such as from home to school or office or for carrying around personal data that the user typically wants to access in a variety of places. The near-ubiquity of USB support on modern computers means that such a drive will work in most places. A drawback to the small size is that they are easy to misplace, leave behind, or otherwise lose, leading to the practice of attaching them to keychains or necklaces.

Flash drives are also a relatively dense form of storage, where even the cheapest will store dozens of floppy disks worth of data. Many can hold more data than a CD (700 MB). Top of the line flash drives can store more data than a double-sided dual-layer DVD - even 64 GB and more.

Flash drives implement the USB mass storage device class, meaning that most modern operating systems can read and write to flash drives without any additional device drivers. The flash drives present a simple block-structured logical unit to the host operating system, hiding the individual complex implementation details of the various underlying flash memory devices. The operating system can use whatever type of filesystem or block addressing scheme it wants. Some computers have the ability to boot up from flash drives.

Flash drives are much more tolerant of abuse than mechanical drives, but can still be damaged or have data corrupted by severe physical impacts. Improperly wired USB ports can also destroy the circuitry of a flash drive, a danger in home-built desktop PCs.

Some flash drives can retain their memory after being submerged in water, even through a machine wash. Leaving the flash drive out to dry completely before allowing current to run through it has been known to result in a working drive with no future problems.


Weaknesses

Like all flash memory devices, flash drives can sustain only a limited number of write and erase cycles before failure. Mid-range flash drives under normal conditions will support several hundred thousand cycles, although write operations will gradually slow as the device ages. This should be a consideration when using a flash drive to run application software or an operating system. To address this, as well as space limitations, some developers have produced special versions of operating systems (such as Linux) or commonplace applications (such as Mozilla Firefox) designed to run from flash drives. These are typically optimized for size and configured to place temporary or intermediate files in the computer’s main RAM rather than store them temporarily on the flash drive.

Most USB flash drives do not include a write-protect mechanism, although some have a switch on the housing of the drive itself to keep the host computer from writing or modifying data on the drive. Write-protection makes a device suitable for repairing virus-contaminated host computers without risk of infecting the USB flash drive itself.

In some USB flash drives, the USB interface is bigger than the storing body.



• Future developments

Semiconductor corporations have worked to reduce the cost of the components in a flash drive by integrating various flash drive functions in a single chip, thereby reducing the part-count and overall package cost.

Flash drive capacities on the market are continuously increasing. As of 2006, 64 MB and smaller capacity flash memory has been largely discontinued, and 128 MB capacity flash memory is being phased out. High-speed USB is now a standard for modern flash drives and capacities of up to 8GB are common.