USB flash drive

USB flash drives are NAND-type flash memory data storage devices integrated with a USB (universal serial bus) connector. They are typically small, lightweight, removable and rewritable. (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, are more reliable due to their lack of moving parts, and have a more durable design. Additionally, it has become increasingly common for computers to ship without floppy disk drives. USB ports, on the other hand, appear on almost every current mainstream PC and laptop. These types of drives use the USB mass storage standard, supported natively by modern operating systems such as Windows, Mac OS X, Linux, and other Unix-like systems.
With nothing being mechanically driven in a flash drive, the name is something of a misnomer. It is called a "drive" because it appears to the computer operating system (and the user) in a manner identical to a mechanical disk drive, and is accessed in the same way.
A flash drive consists of a small printed circuit board typically in a plastic or metal casing and more recently in rubber casings to increase their robustness. This makes the drive sturdy enough to be carried about in a pocket, for example as a key fob, or on a lanyard. Only the USB connector protrudes, and it is typically protected either by a removable cap or by retracting into the body of the drive. 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 as 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 for 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.

First invention and sale

Several companies claim to have invented the USB flash drive. Trek was the first company to sell a USB flash drive (ThumbDrive) in early 2000. However, their patent does not describe the USB flash drive; rather, it describes a very broad family of storage devices which could include the USB flash drive.
M-Systems (acquired by SanDisk in November 2006) had been working on a USB flash drive since 1998. They registered the domain www.diskonkey.com on October 12, 1999,[2indicating their intention to sell a product. In 2000 Dan Harkabi joined the M-Systems team and led the development of DiskOnKey. The industrial design was done by Ziba and the product won the IDEA award in 2001. M-Systems' patent rigorously describes the USB flash drive and its implementation.
An IBM invention disclosure RPS8-1999-0201 (Sept. 99) by Shimon Shmueli et al. is the earliest known document to describe the USB-FD accurately and completely, and only the USB-FD. M-Systems manufactured the DiskOnKey for IBM, which in late 2000 was the first to sell the product in North America. The IBM 8 MB USB Memory Key became available December 15, 2000. Shmueli later founded KeyNetica, the first company to patent and develop the concept that mobile and smart storage devices are all one needs for mobile computing. Current implementors of the concept are U3 (part of SanDisk, which also owns the original KeyNetica patent) and Ceedo.

Second generation

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 currently available use a dual channel controller, although they still fall considerably short of the transfer rate possible from a current generation hard disk, or the maximum high speed USB throughput.
Typical overall file transfer speeds are about 3 MB/s. The highest current overall file transfer speeds are about 10-30 MB/s. Older, "full speed" 12 Mbit/s devices are limited to a maximum of about 1 MB/s.

Components

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.

Essential components

There are typically four 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 levelling, or wear balancing. 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.

Additional components

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 the 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 elaborate housings, which are often bulky and make the drive 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. Such devices may only carry the USB logo if sold with a separate extension cable.
USB flash drives have been integrated into other tools commonly carried on one's person such as a watches, pens, and even the Swiss Army Knife; others have been fitted with novelty cases such as toy cars or LEGO bricks. The small size, robust nature and relatively low price of USB flash drives makes them an increasingly popular peripheral for case modding.
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 1.0 standard.