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 30 g (1 oz). Storage capacities
in 2009 can be as large as 256 GB with steady improvements in size
and price per capacity. Some allow 1 million write or erase cycles
and have a 10-year data retention cycle.
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.
Until approximately 2005, most desktop and laptop computers were
supplied with floppy disc drives, but most recent equipment has
abandoned floppy disk drives in favor of USB ports.
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 and can be
read by most other systems such as the Playstation 3.
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. 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.
Uses
Personal data transport
The most common use of flash drives is to transport and store personal
files such as documents, pictures and videos. Individuals also store
medical alert information on MedicTag flash drives for use in
emergencies and for disaster preparation.
Secure storage of data, application and software files
With wide deployment(s) of flash drives being used in various
environments (secured or otherwise), the issue of data and information
security remains of the utmost importance. The use of biometrics and
encryption is becoming the norm with the need for increased security for
data; OTFE systems such as FreeOTFE and TrueCrypt are particularly
useful in this regard, as they can transparently encrypt large amounts
of data. In some cases a Secure USB Drive may use a hardware-based
encryption mechanism that uses a hardware module instead of software for
strongly encrypting data.
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.
They are also used as a means to transfer recovery and antivirus
software to infected PCs, allowing a portion of the host machine's data
to be archived. As the drives have increased in storage space, they have
also replaced the need to carry a number of CD ROMs and installers which
were needed when reinstalling or updating a system.
Application carriers
Flash drives are used to carry applications that run on the host
computer without requiring installation. While any standalone
application can in principle be used this way, many programs store data,
configuration information, etc. on the hard drive and registry of the
host computer
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 not to leave
any data on the host machine. U3 also provides a software framework for
independent software vendors 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.
Similarly, other application virtualization solutions and portable
application creators, such as VMware ThinApp (for Windows) or RUNZ (for
Linux) can be used to run software from a flash drive without installation.
A wide 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, can be found in the list of
portable software.
Advantages
Data stored on flash drives are impervious to scratches and dust, and
flash drives are mechanically very robust making them suitable for
transporting data from place to place and keeping it readily at hand.
Most personal computers support USB as of 2009.
Flash drives also store data densely compared to many removable media.
In mid-2009, 256 GB drives became available, with the ability to hold
many times more data than a DVD or even a Blu-ray disc.
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 even 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 removable storage devices tested, using
optical or magnetic technologies, were destroyed.
Comparison with other portable storage
Tape
The applications of current data tape cartridges hardly overlap those of
flash drives: the drives and media are expensive, have very high
capacity and very fast transfer speeds, and store data sequentially.
While disk-based backup is the primary medium of choice for most
companies, tape backup is still popular for taking data off-site for
worst-case scenarios. See LTO tapes.
Floppy disk
Size comparison of a flash drive and a 3.5-inch floppy disk
Floppy disks are rarely fitted to modern computers and are obsolete for
normal purposes, although internal and external drives can be fitted if
required. Floppy disks may be the method of choice for transferring data
to and from very old computers without USB or booting from floppy disks,
and so they are sometimes used to change the firmware on, for example,
BIOS chips. Devices with removable storage like older Yamaha music
keyboards are also dependent on floppy disks, which require computers to
process them. Newer devices are built with USB flash drive support.
Optical media
The various writable and rewritable forms of CD and DVD are portable
storage media supported by the vast majority of computers as of 2008.
CD-R, DVD-R, and DVD+R can be written to only once, RW varieties up to
about 1,000 erase/write cycles, while modern NAND-based flash drives
often last for 500,000 or more erase/write cycles. DVD-RAM discs are
the most suitable optical discs for data storage involving much rewriting.
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
over optical media.
SD Drives
Secure Digital (SD) is a non-volatile memory card format developed for use in portable devices. Currently it is widely used in digital cameras, digital camcorders, handheld computers,PDAs, media players, mobile phones, GPS receivers, and video games. Standard SD card capacities range from 1 MB to 4 GB. The capacity range for high capacity SDHC cards overlap, beginning at 4 GB but reaching as high as 32 GB as of mid-2009. The SDXC (eXtended Capacity), a new specification announced at the 2009 Consumer Electronics Show, allows for up to 2 TB capacity cards.
The format has proven very popular. Changes to the interface of the established format have made some older devices designed for standard SD cards (≤4GB) unable to handle newer formats such as SDHC (≥4GB). All SD-cards have the same physical shape and form factor however, which causes confusion for many consumers.
Design and implementation
SD cards are based on the older MultiMediaCard (MMC) format, but have a number of differences:
- • The SD card is asymmetrically shaped in order not to be inserted upside down, while an MMC would go in most of the way but not make contact if inverted.
- • Most SD cards are physically thicker than MMCs. SD cards generally measure 32 mm × 24 mm × 2.1 mm, but as with MMCs can be as thin as 1.4 mm if they lack a write-protect switch; such cards, called "Thin SD", are described in the SD specification, but they are non-existent or rare in the market as devices that would require a thinner card are usually utilising the smaller (and thinner) versions of SD: miniSD or microSD.
- • The card's electrical contacts are recessed beneath the surface of the card, protecting them from contact with a user's fingers.
- • SD cards typically have transfer rates in the range of 10-20 MB/s, but this number is subject to change, due to recent improvements to the MMC standard
Devices with SD slots can use the thinner MMCs, but standard SD cards will not fit into the thinner MMC slots.miniSD cards can be used directly in SD slots with a simple passive adapter, since the cards differ in size and shape but not electrical interface. With an active electronic adapter, SD cards can be used in CompactFlash or PC card slots. Some SD cards include a USB connector for compatibility with desktop and laptop computers, and card readers allow SD cards to be accessed via connectivity ports such as USB, FireWire, and the parallel printer port. SD cards can also be accessed via a floppy disk drive with a FlashPath adapter.
Speeds
here are different speed grades available, measured the same as CD-ROMs, in multiples of 150 kB/s (1x = 150 kB/s). Basic cards transfer data up to six times (6x) the data rate of the standard CD-ROM speed (900 kB/s vs. 150 kB/s).
Note that maximum read speed and maximum write speed may be different. Maximum write speed typically is lower than maximum read speed. Some digital cameras require high-speed cards (write speed) to record video smoothly or capture multiple still photographs in rapid succession. This requires a certain sustained speed, or the video stops recording. For recording, a high maximum speed with a low sustained speed is no better than a low speed card. The 2.0 specification defines speeds up to 200x.
Some manufacturers use the read speed in their X-ratings, while others (Kingston, for example) use write speed.
This table lists common ratings and minimum transfer rates.
| Rating |
Write Speed (MByte/s) |
SD Class |
| 6x |
0.9 |
n/a |
| 10x |
1.5 |
n/a |
| 13x |
2.0 |
2 |
| 26x |
4.0 |
4 |
| 32x |
4.8 |
4 |
| 40x |
6.0 |
6 |
| 66x |
10.0 |
10 |
| 100x |
15.0 |
15 |
| 133x |
20.0 |
20 |
| 150x |
22.5 |
22 |
| 200x |
30.0 |
30 |
| 266x |
40.0 |
40 |
| 300x |
45.0 |
45 |
