USB flash drive
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USB flash drives are NAND-type flash memory data storage devices integrated with a USB interface. They are typically small, lightweight, removable and rewritable. As of November 2006, 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, that use plug-in rather than built-in memory.)
USB flash drives offer potential advantages over other portable storage devices, particularly the floppy disk. They are more compact, generally faster, hold more data, and may be more reliable (due to their lack of moving parts) than floppy disks. These types of drives use the USB mass storage standard, supported natively by modern operating systems such as Linux, Mac OS X, and Windows.
A flash drive consists of a small printed circuit board encased 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.
Most flash drives are active only when powered by a USB computer connection, and require no other external power source or battery power source; they are powered using the limited supply afforded by the USB connection. To access the data stored in a flash drive, the flash drive must be connected to a computer, either by direct connection to the computer's USB port or via a USB hub.
However, even though the USB ports on a computer can power the flash drive, some flash drives can be attached to the USB port on a keyboard, as with a Mac computer. Other flash drives may not receive sufficient power from a USB port on a keyboard, and need to be plugged into a USB port on the computer itself.
The flash drive was first invented in 1998 by Dov Moran, President and CEO of M-Systems Flash Pioneers (Israel). Dan Harkabi, who is now a Vice President at SanDisk, led the development and marketing team at M-Systems. His most significant contribution was his insistence that the product be self-reliant and free of the need to install drivers. Nearly simultaneous development of similar products was undertaken at Netac and at Trek 2000, Ltd. All three companies have similar and disputed patents. IBM was the first North American seller of a USB flash drive, and marketed an 8 MB version of the product in 2001 under the "Memory Key" moniker. IBM later introduced a 16 MB version manufactured by Trek 2000, and returned to M-Systems for the 64 MB version in 2003. Lexar can also lay claim to a pioneering USB flash drive product. In 2000 they introduced a Compact Flash (CF) card having an internal USB function. Lexar offered a companion card reader and USB cable that eliminated the need for a USB hub.
The first flash drives were made by M-Systems and distributed in Europe under the "disgo"  brand in sizes of 8 MB, 16 MB, 32 MB, and 64 MB. These were marketed as "a true floppy-killer", and this design was continued up to 256 MB. Asian manufacturers soon started making their own flash drives that were cheaper than the disgo series.
Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s the specification supports due to technical limitations inherent in NAND flash. The fastest drives available now use a dual channel controller, though still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB 2.0 throughput.
Typical overall file transfer speeds are about 3 MBytes/s. The highest current overall file transfer speeds are about 10-25 MByte/s. Older, "full speed" 12 Mbit/s devices are limited to a maximum of about 1 MByte/s.
Flash drives have become iconic as a sort of "fashion statement" , much like the iPod's white ear bud headphones.
One end of the device is fitted with a single male type-A USB connector. Inside the plastic casing is a small printed circuit board. Mounted on this board is some simple power circuitry and a small number of surface-mounted integrated circuits (ICs). Typically, one of these ICs provides an interface to the USB port, another drives the onboard memory, and the other is the flash memory.
There are typically three parts to a flash drive:
- Male type-A USB connector - provides an interface to the host computer.
- USB mass storage controller - implements the USB host controller and provides a linear interface to block-oriented serial flash devices while hiding the complexities of block-orientation, block erasure, and wear balancing, or wear levelling. The controller contains a small RISC microprocessor and a small amount of on-chip ROM and RAM.
- NAND flash memory chip - stores data. NAND flash is typically also used in digital cameras.
- Crystal oscillator - produces the device's main 12 MHz clock signal and controls the device's data output through a phase-locked loop.
The typical device may also include:
- Jumpers and test pins - for testing during the flash drive's manufacturing or loading code into the microprocessor.
- LEDs - indicate data transfers or data reads and writes.
- Write-protect switches - indicate whether the device should be in "write-protection" mode.
- Unpopulated space - provides space to include a second memory chip. Having this second space allows the manufacturer to develop only one printed circuit board that can be used for more than one storage size device, to meet the needs of the market.
- USB connector cover or cap - reduces the risk of damage due to static electricity, and improves overall device appearance. Some flash drives do not feature a cap, but instead have retractable USB connectors. Other flash drives have a "swivel" cap that is permanently connected to the drive itself and eliminates the chance of losing the cap.
- Transport aid - In some cases, the cap or main body contains a hole suitable for connection to a key chain or lanyard or to otherwise aid transport and storage of the USB flash device.
Size and style of packaging
Some manufacturers differentiate their products by using unnecessarily elaborate housings. An example is some of Lexar's Jump Drives which are often bulky and difficult to connect to the USB port. Because the USB port connectors on a computer housing are often closely spaced, plugging a flash drive into a USB port may block an adjacent port.
Recently, USB flash drives have been integrated into other things such as a watch or a pen.
Overweight or ill-fitting flash drive packaging can cause disconnection from the host computer. This can be overcome by using a short USB to USB (male to female) extension cable to relieve tension on the port. Such cables are USB-compatible, but do not conform to the USB standard.  
- Personal data transport
- The most common use of flash drives is by individuals to transport and store personal files such as documents, pictures and video.
- Computer repair
- Flash drives enjoy notable success in the PC repair field as a means to transfer recovery and antivirus software to infected PCs, while allowing a portion of the host machine's data to be archived in case of emergency.
- System administration
- Flash drives are particularly popular among system and network administrators, who load them with configuration information and software used for system maintenance, troubleshooting, and recovery.
- Application carriers
- Flash drives are used to carry applications that run on the host computer without requiring installation. U3, backed by flash drive vendors, offers an API to flash drive-specific functions. A free and open-source software platform known as Portableapps has also been developed to allow U3-like functionality on non-U3 drives. airWRX is an application framework that runs from a flash drive and turns its PC host and other nearby PCs into a multi-screen, web-like work environment. The Mozilla Firefox browser has a configuration for flash drives, as does Opera.
- Audio players
- Many companies make solid state digital audio players in a small form factor, essentially producing flash drives with sound output and a simple user interface. Probably the best-known of these has been Apple Computer's iPod shuffle, and the Creative Labs MuVo.
- To boot operating systems
- In a way similar to that used in LiveCD, one can launch any operating system from a bootable flash drive, known as a LiveUSB.
- In arcades
- In the arcade game In the Groove and more commonly In The Groove 2, flash drives are used to transfer high scores, screenshots, dance edits, and combos throughout sessions. While use of flash drives is common, the drive must be Linux compatible, causing problems for some players. Data used can then be uploaded to Groovestats.
Strengths and weaknesses
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.
Flash drives are also a relatively dense form of storage, where even the cheapest will store dozens of floppy disks worth of data. Some can hold more data than a CD (700 MB). Top of the line flash drives can store more data than a DVD (4.7 GB).
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.
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 memory rather than store them temporarily on the flash drive.
Most USB flash drives do not include a write-protect mechanism. Such a switch on the housing of the drive itself would keep the host computer from writing or modifying data on the drive. Write-protection would make a device suitable for repairing virus-contaminated host computers without infecting the USB flash drive itself.
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.
Comparison to other portable memory forms
Flash storage devices are best compared to other common, portable, swappable data storage devices: floppy disks, Zip disks, miniCD / miniDVD, CD-R/CD-RW and DVD-RW discs. 3.5 inch floppy disks and Iomega Zip disks are still available as of mid-2006, despite their declining popularity.
Floppy disks were the first publicly-popular method of file transport, but are becoming obsolete due to their low capacity, low speed, and low durability. Virtually all new computers include USB ports, and many of them are now sold without a floppy drive, the Apple iMac being the first to ship this way. USB external floppy disk drives are available. Floppy disks are still in use because of their low cost and ease of use with older systems. Attempts to extend the floppy standard (such as the Imation SuperDisk) were not successful because of a reputation for unreliability and the lack of a single standard for PC vendors to adopt.
The Iomega Zip drive enjoyed some popularity, but never reached the point of ubiquity in computers. Also, the larger sizes of Zip - now up to 750 MB - cannot be read on older drives. Unless one were to carry around an external drive, their usefulness as a means of moving data was rather limited. The cost per megabyte was fairly high, with individual disks often priced at US$10 or higher. Because the material used for creating the storage medium in Zip disks is similar to that used in floppy disks, Zip disks have a higher risk of failure and data loss. Larger removable storage media, like Iomega's Jaz drive, had even higher costs, both in drives and in media, and as such were never feasible as a floppy alternative.
CD-R and CD-RW are swappable storage media alternatives. Unlike Zip and floppy drives, DVD and CD recorders are now common in personal computer systems. CD-Rs can only be written to once, and the more expensive CD-RWs are only rated up to 1,000 erase/write cycles, whereas modern NAND-based flash drives often last for 500,000 or more erase/write cycles. Optical storage devices are also usually slower than their flash-based counterparts. Compact discs with an 11.5 cm diameter can also be inconveniently large and, unlike flash drives, cannot fit into a pocket or hang from a keychain. Smaller CDs are available, but these are unusual. There is also no standard file system for rewriteable optical media; packet-writing utilities like DirectCD and InCD exist, but produce discs that are not universally readable, despite claiming to be based on the UDF standard. The upcoming Mount Rainier standard addresses this shortcoming in CD-RW media, but is still not supported by most DVD and CD recorders or major operating systems. As a result, CDs are a good way to record a great deal of information cheaply, but not good for making ongoing small changes to a large collection of information; this is a major advantage of flash drives.
Some flash drives feature encryption of the data stored on them, generally using full disk encryption below the filesystem. This prevents an unauthorized person from accessing the data stored on it. The disadvantage is that the drive is accessible only in the minority of computers which have compatible encryption software, for which no portable standard is widely deployed.
Some encryption applications allow running without installation. The executable files can be stored on the USB drive, together with the encrypted file image. The encrypted partition can be accessed on any computer running Microsoft Windows. Other flash drives allow the user to configure secure and public partitions of different sizes. Executable files for Windows, Macintosh, and Linux are usually included on the drive.
Newer flash drives support biometric fingerprinting to confirm the user's identity. As of mid-2005, this was a relatively costly alternative to standard password protection offered on many new USB flash storage devices.
Some manufacturers deploy physical authentication tokens in the form of a flash drive. These are used to control access to a sensitive system by containing encryption keys or, more commonly, communicating with security software on the target machine. The system is designed so the target machine will not operate except when the flash drive device is plugged into it. Some of these "PC lock" devices also function as normal flash drives when plugged into other machines.
All such forms of data protection security involve an increased risk of loss of access to the data by the legitimate owner/user.
Flash drives present a significant security challenge for large organizations. Their small size and ease of use allows unsupervised visitors or unscrupulous employees to smuggle confidential data out with little chance of detection. Equally, corporate and public computers alike are vulnerable to attackers connecting a flash drive to a free USB port and using malicious software such as rootkits or packet sniffers. To prevent this, some organizations forbid the use of flash drives, and some computers are configured to disable the mounting of USB mass storage devices by ordinary users, a feature introduced in Windows XP Service Pack 2; others use third-party software to control USB usage. In a lower-tech security solution, some organizations disconnect USB ports inside the computer or fill the USB sockets with epoxy.
Recently, "USB flash drive" or simply "UFD" has emerged as the de facto standard term for these devices. Many major manufacturers (SanDisk, Lexar, Kingston) and resellers use the term UFD to describe them. However, the myriad of different brand names and terminology used, in the past and currently, makes UFDs more difficult for manufacturers to market and for consumers to research. Some commonly used names are actually trademarks of particular companies e.g. 'disgo'.
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.
In efforts to focus on increasing capacities, 64 MB and smaller capacity flash memory has been largely discontinued, and 128 MB capacity flash memory is being phased out. Kanguru has recently released a 64 GB flash memory drive that uses USB 2.0 and claims 10 years worth of information preservation. 
Lexar is attempting to introduce a USB flash card  , which would be a compact USB flash drive intended to replace various kinds of flash memory cards.
SanDisk has introduced a new technology to allow controlled storage and usage of copyrighted materials on flash drives, primarily for use by students. This technology is termed FlashCP.
- In 2004, the German punk band WIZO was the first musical group to release music in MP3 format on a USB drive, titled the WIZO Stick-EP. 
- In the films The Recruit and Collateral, thumb drives play an important role in the plot.
- 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 has been known to result in a working drive with no future problems.
- Isolinear optical chips, fictional devices similar to USB drives in design and function, were featured regularly in Star Trek: The Next Generation over 10 years before their real world counterparts were invented.
- In The Running Man (film) (1987), Amber Mendez (Maria Conchita Alonso) searches for video of "The Bakersfield Massacre" within the studio where she works. The video is stored on small flash memory card media (similar to CompactFlash, but in a matte black casing). Due to the advances of audio/video codec technology it is possible to store high quality video on such media today.
- On South Park a 1 GB flash drive was used to hold the Sword of a Thousand Truths which was previously removed from World of Warcraft in the episode Make Love, Not Warcraft.
- External hard drive
- File synchronization
- List of backup software
- List of portable software
- LiveDistro - Computer Operating System on a Flash Drive
- Live USB
- Pocket hard drive
- USB Flash Drive Alliance
- USB Mass Storage Device
- ^ Disgo Website. Retrieved on 2006-08-01.
- ^ "From Storage, a New Fashion", The New York Times. Retrieved on 2006-08-01.
- ^ Opera@USB : Portable Opera for free. Retrieved on 2006-08-01.
- ^ "WIZO-STICK-EP"-USB-MEMORY-FLASH-STICK. Retrieved on 2006-08-01.
- ^ Kingmax Super Stick. Retrieved on 2006-08-01.
- Singapore firm wins patent on thumb drive
- Creating a RAID array with Apple Shuffle Flash units
- Experimenting with creating a USB flash drive RAID array in Linux and Windows.
- Encrypted thumb drive and autoplay howto - Open source tools and a very clear walkthrough
- Installing USB Flash Drives on Windows 98 - Generic driver download for Windows 98 SE to allow USB flash drives to work.