A USB flash drive consists of a NAND-type flash memory data storage device integrated with a USB (universal serial bus) interface. USB flash drives are typically removable and rewritable, much shorter than a floppy disk (1 to 4 inches or 2.5 to 10 cm), and weigh less than 2 ounces (56 g). Storage capacities typically range from 64 MB to 64 GB with steady improvements in size and price per gigabyte. Some allow 1 million write or erase cycles and have 10-year data retention, connected by USB 1.1 or USB 2.0. 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 have a more compact shape, operate faster, hold much more data, have a more durable design, and operate more reliably due to their lack of moving parts. 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. USB drives with USB 2.0 support can also operate faster than an optical disc drive, while storing a larger amount of data in a much smaller space.
Nothing actually moves in a flash drive: the term drive persists because computers read and write flash-drive data using the same system commands as for a mechanical disk drive, with the storage appearing to the computer operating system and user interface as just another drive.
A flash drive consists of a small printed circuit board protected inside a plastic, metal, or rubberised case, robust enough for carrying with no additional protection — in a pocket or on a key chain, for example. The USB connector is protected by a removable cap or by retracting into the body of the drive, although it is not liable to be damaged if exposed. Most flash drives use a standard type-A USB connection allowing plugging into a port on a personal computer.
Flash memory combines a number of older technologies, with the low cost, low power consumption and small size made possible by recent advances in microprocessor technology. The memory storage is based on earlier EPROM and EEPROM technologies. These had very limited capacity, were very slow for both reading and writing, required complex high-voltage drive circuitry, and could only be re-written after erasing the entire contents of the chip.
Hardware designers later developed EEPROMS with the erasure region 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 memory systems with serially accessed storage 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).
Computers access modern flash memory systems 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 players can also be used as a conventional flash drive.
Trek Technology and IBM began selling the first USB flash drives commercially in 2000. Singaporean company Trek Technology sold a model dubbed the "ThumbDrive," and IBM marketed the first such drives in North America, with its product the "DiskOnKey" (which was manufactured by M-Systems). IBM's USB flash drive became available December 15, 2000, and had a storage capacity of 8 MB, more than five times the capacity of the (at the time) commonly used floppy disks.
In 2000 Lexar introduced a Compact Flash (CF) card with a USB connection, and a companion card read/writer and USB cable that eliminated the need for a USB hub.
In 2004 Trek Technology brought several lawsuits against other USB flash drive manufacturers and distributors in an attempt to assert its patent rights to the USB flash drive. A court in Singapore ordered competitors to cease selling similar products that would be covered by Trek's patent, but a court in the United Kingdom revoked one of Trek's patents in that country.
Typical overall file transfer speeds vary considerably, and should be checked before purchase; speeds may be given in megabytes or megabits per second. Typical fast drives claim to read at up to 30 megabytes/s (MB/s) and write at about half that. Older "USB full speed" 12 megabit/s devices are limited to a maximum of about 1 MB/s.
Drives typically use the USB mass storage device class to communicate with the host.
Internals of a typical USB flash drive
|2||USB mass storage controller device|
|4||Flash memory chip|
|8||Space for second flash memory chip|
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 commonly-carried items such as 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, robustness and cheapness of USB flash drives make them an increasingly popular peripheral for case modding.
Heavy or bulky flash drive packaging can make for unreliable operation when plugged directly into a USB port; this can be relieved by a USB extension cable. Such cables are USB-compatible, but do not conform to the USB 1.0 standard.
Most flash drives ship preformatted with the FAT or FAT 32 file system. The ubiquity of this file system allows the drive to be accessed on virtually any host device with USB support. Also, standard FAT maintenance utilities (e.g. ScanDisk) can be used to repair or retrieve corrupted data. However, because a flash drive appears as a USB-connected hard drive to the host system, the drive can be reformatted to any file system supported by the host operating system.
Flash drives can be defragmented, but this brings little advantage as there is no mechanical head slowed down by having to move from fragment to fragment (flash drives often have very large internal sector size, especially when erasing so defragmenting means accessing fewer sectors to erase a file). Defragmenting shortens the life of the drive by making many unnecessary writes.
Some file systems are designed to distribute usage over an entire memory device without concentrating usage on any part (e.g., for a directory); this prolongs life of simple flash memory devices. USB flash drives, however, have this functionality built into the controller to prolong device life, and use of such a file system brings less advantage.
The U3 company works with drive makers (parent company SanDisk as well as others) to deliver custom versions of applications designed for Microsoft Windows from a special flash drive; U3-compatible devices are designed to autoload a menu when plugged into a computer running Windows. Applications must be modified for the U3 platform and not to leave any data on the host machine. U3 also provides a software framework for ISVs interested in their platform.
Ceedo is an alternative product with the key difference that it does not require Windows applications to be modified in order for them to be carried and run on the drive.
A range of portable applications which are all free of charge and able to run off a computer running Windows without storing anything on the host computer's drives or registry is available from portableapps.com; unlike U3 programs which run from a special U3-compatible USB stick, the PortableApps menu will run from a standard device, but does not use the Windows AutoRun feature.
While a Live USB could be used for general-purpose applications, size and memory wear make them poor choices compared to alternatives. They are more suited to special-purpose or temporary tasks, such as:
Many of the smallest players are powered by a permanently fitted rechargeable battery, charged from the USB interface.
Usually, such drives will be custom-stamped with a company's logo, as a form of advertising to increase mind share and brand awareness. The drive may be blank drive, or preloaded with graphics, documentation, web links, Flash animation or other multimedia, and free or demonstration software. Some preloaded drives are read-only; others are configured with a read-only and a writeable partition. Dual-partition drives are more expensive.
Flash drives can be set up to autorun stored presentations, websites and articles immediately on insertion of the drive by saving a file called autorun.inf with an appropriate shell script in the root directory of the drive. Autoloading this way does not work on all computers; the U3 drives described above load more reliably.
It is also easy to lose these small devices, and easy for people without a right to data to take illicit backups.
Flash drives also store data relatively densely compared to many removable media. In mid-2008, 64 GB drives became available, with the ability to hold many times more data than a DVD.
Compared to hard drives, flash drives use little power, have no fragile moving parts, and for low capacities are small and light.
Flash drives implement the USB mass storage device class so that most modern operating systems can read and write to them without installing 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 any file system or block addressing scheme. Some computers can boot up from flash drives.
Some flash drives retain their memory after being submerged in water , even through a machine wash, although this is not a design feature and not to be relied upon. 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. Channel Five's Gadget Show cooked a flash drive with propane, froze it with dry ice, submerged it in various acidic liquids, ran over it with a jeep and fired it against a wall with a mortar. A company specializing in recovering lost data from computer drives managed to recover all the data on the drive. All data on the other removal storage devices tested, using optical or magnetic technologies, were destroyed.
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.
A drawback to the small size is that they are easily misplaced, left behind, or otherwise lost. This is a particular problem if the data they contain are sensitive (see data security). As a consequence, some manufacturers have added encryption hardware to their drives -- although software encryption systems achieve the same thing, and are universally available for all USB flash drives. Others just have the possibility of being attached to keychains, necklaces and lanyards.
Compared to other portable storage device, for example external hard drives, USB flash drives have a high price per unit of storage and are only available in comparatively small capacities; but in the smaller capacities (4 GB and less), USB flash drives are much less expensive per unit of storage than the hard drives they have replaced.
Optical storage devices are among the cheapest methods of mass data storage after the hard drive. They are slower than their flash-based counterparts. Standard 12 cm optical discs are larger than flash drives and more subject to damage. Smaller optical media do exist, such as business card CD-Rs which have the same dimensions as a credit card, and the slightly less convenient but higher capacity 8 cm recordable CD/DVDs. The small discs are more expensive than the standard size, and do not work in all drives.
Universal Disk Format (UDF) version 1.50 and above has facilities to support rewritable discs like sparing tables and virtual allocation tables, spreading usage over the entire surface of a disc and maximising life, but many older operating systems do not support this format. Packet-writing utilities such as DirectCD and InCD are available but produce discs that are not universally readable (although based on the UDF standard). The Mount Rainier standard addresses this shortcoming in CD-RW media by running the older file systems on top of it and performing defect management for those standards, but it requires support from both the CD/DVD burner and the operating system. Many drives made today do not support Mount Rainier, and many older operating systems such as Windows XP and below, and Linux kernels older than 2.6.2, do not support it (later versions do). Essentially CDs/DVDs are a good way to record a great deal of information cheaply and have the advantage of being readable by most standalone players, but they are poor at making ongoing small changes to a large collection of information; flash drives' ability to do this is their major advantage.
Particularly with the advent of USB, external hard disks have become widely available and relatively inexpensive. As drive capacity increases, external hard disk drives cost less per megabyte than flash drives, and are available in much larger capacities. Some hard drives support alternative and faster interfaces than USB 2.0 (e.g. IEEE 1394 and eSATA). For writes and consecutive sector reads (for example, from an unfragmented file), most hard drives can provide a much higher sustained data rate than current NAND flash memory.
Unlike solid-state memory, hard drives are susceptible to damage by shock, e.g., a short fall, have limitations on use at high altitude, and, like all magnetic media, are vulnerable when exposed to large magnetic fields, although shielded by their casing. Hard drives are usually larger and heavier than flash drives, although not necessarily when compared per unit storage. Hard disks also suffer from file fragmentation which can significantly reduce performance. Some file systems are resistant to fragmentation, but all general purpose file systems suffer from some degree of fragmentation, and some consequent degradation of performance.
All USB flash drives can have their contents encrypted using 3rd party disk encryption software such as FreeOTFE and TrueCrypt, which can be used without installation. The executable files can be stored on the USB drive, together with the encrypted file image. The encrypted partition can then be accessed on any computer running the correct operating system, although may require administrative rights on the host computer to access data.
Other flash drives allow the user to configure secure and public partitions of different sizes, and offer hardware encryption.
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. Most fingerprint scanning drives rely upon the host operating system to validate the fingerprint via a software driver, often restricting the drive to Microsoft Windows computers. However, there are USB drives with fingerprint scanners use controllers that allow access to protected data without any authentication.
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.
Some organizations simply forbid the use of flash drives altogether, 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.
Flash drive capacities on the market increase continually. few manufacturers continue to produce models of 256 MB and smaller; and many have started to phase out 512 MB capacity flash memory. High-speed has become a standard for modern flash drives and capacities of up to 64 GB have come on the market.
Lexar is attempting to introduce a USB FlashCard , which would be a compact USB flash drive intended to replace various kinds of flash memory cards. Pretec introduced a similar card, which also plugs into every USB port, but is just one quarter the thickness of the Lexar model SanDisk has a product called SD Plus, which is a SecureDigital card with a USB connector.
SanDisk has also 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.