A USB flash drive consists of flash memory data storage device integrated with a USB (Universal Serial Bus) 1.1 or 2.0 interface. USB flash drives are typically removable and rewritable, much smaller than a floppy disk, and most weigh less than 1 ounce (30 g).[1] Storage capacities can range from 64 MB to 256 GB[2] with steady improvements in size and price per capacity. Some allow 1 million write or erase cycles[3][4] and have 10-year data retention,[5].
USB flash drives are often used for the same purposes as floppy disks were. They are smaller, faster, have thousands of times more capacity, and are more durable and reliable due to their lack of moving parts. Since floppy discs were developed until about 2005 most PC and laptop computers were supplied with floppy disc drives, but most recent equipment has several USB ports but does not support floppy disks.
Flash 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 store more data and transfer faster than a much larger optical disc drive.
Nothing moves mechanically 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.[4] Flash drives are very robust mechanically, and can withstand anything that does not actually break the circuit board or connector.
A flash drive consists of a small printed circuit board carrying the circuit elements and a USB connector, insulated electrically and protected inside a plastic, metal, or rubberized case which can be carried in a pocket or on a key chain, for example. The USB connector may be protected by a removable cap or by retracting into the body of the drive, although it is not likely to be damaged if unprotected. Most flash drives use a standard type-A USB connection allowing plugging into a port on a personal computer, but drives for other interfaces also exist.
Most USB flash drives derive their power from the USB connection, and do not require a battery. Some devices which combine the functionality of a digital audio player with flash-drive-type storage require a battery for the player function.
Technology
Flash memory combines a number of older technologies, with lower cost, lower power consumption and small size made possible by recent[update] 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 copying the entire field into an off-chip buffer memory, erasing the field, modifying the data as required in the buffer, and re-writing it into the same field. 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 made semiconductor 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 requires far fewer electrical connections for the memory chips than does parallel access, which has simplified the manufacture of multi-gigabyte dives.
Computers access modern[update] 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 and a battery 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, for storing files of any type.
History
First commercial product
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 the Israeli company M-Systems). IBM's USB flash drive became available on December 15, 2000,[6] and had a storage capacity of 8 MB, more than five times the capacity of the commonly used floppy disks (floppy disks having a capacity of 1.44MB).
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.
On July 24, 2002 Netac Technology was granted a highly-contested Chinese patent for the USB flash drive.[7]
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[8] that would be covered by Trek's patent, but a court in the United Kingdom revoked[9] one of Trek's patents in that country.
Second generation
Modern flash drives have USB 2.0 connectivity. However, they do not currently use the full 480 Mbit/s (60MB/s) the USB 2.0 Hi-Speed 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 vary considerably, and should be checked before purchase. Speeds may be given in Mbyte per second, Mbit per second, or optical drive multipliers such as "180X" (180 times 150 KiB 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 Mbit/s devices are limited to a maximum of about 1 MB/s.
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