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A keydrive is a small removable data storage device that uses flash memory and a USB connector.
Such a device is also known under several other names:
keychain drive
pen drive
pocket drive
thumb drive
USB flash drive
USB flash memory drive
USB key
USB memory key
USB stick
Keydrives typically consist of a small plastic package (around 30 to 40 mm long). One end is fitted a single male type-A USB connector. Inside the plastic package is a small, highly cost-engineered, printed circuit board. On this is mounted some simple power circuity and a single surface-mounted integrated circuit (IC). This IC incorporates a flash memory device, related read/write/erase logic, some elementary block-mapping and address-decode logic, and the USB slave-device interface logic. Many keydrives also feature an LED activity indicator. In operation, keydrives are plugged into a normal type-A USB socket, either on a computer or on a USB hub. Keydrives take their power from the USB connection of the PC to which they are connected and do not need batteries. They are impervious to the scratches and dust that have plagued previous forms of portable storage media like compact discs and floppy disks. Most are lightweight and small - about the size of a key or a thumb. They are popular with people who carry data between home and school or work and are quickly replacing the floppy disk as a portable data device. They are also widely used by system and network administrators, who load them with software for troubleshooting and recovery.
Historically, keydrive capacity has ranged from a few megabytes in size up to a few gigabytes, although some computers have trouble reading and writing to devices that have more than 2 GB of storage. In 2003 most keydrives ran at the USB 1.0/1.1 speeds of 1.5 Mbit/s or 12 Mbit/s. 2004 saw the release of newer USB keys featuring USB 2.0 interfaces. Although USB 2.0 tops out at 480 Mbits/s, these keydrives are limited by the bandwidth of the underlying flash memory device, with maximum read speeds of around 100 Mbits/s and write speeds a little slower. In ideal conditions, the flash memory in the drives can retain data for 10 years.
Like all flash memory devices, keydrives can sustain only a limited number of write/erase cycles before failure. In normal use, mid-range keydrives currently on the market will support several million cycles, although write operations will gradually slow as the device ages. This should be a consideration when using a keydrive as a hard drive to run software or an operating system.
Some keydrives have a physical "write protect" switch so they may be plugged with impunity into a system that might harbor a virus or worm (when set to write protected).
Keydrives implement the USB "mass storage device" interface, meaning that most modern operating systems can read and write to keydrives without any additional device drivers. This means that Linux (kernel versions from 2.4, and 2.2 with patches), Mac OS X, Microsoft's Windows Me, Windows 2000, Windows XP, and Windows Server 2003 are all natively able to access keydrives (most of these present the keydrive to the user as simply another removable drive). Windows 98, which was one of the first OSes with much support for USB, needs specialized software for each different type of USB storage device it encounters. Normally only Windows 98 and Windows 95 OSR2 or release version 2 supports USB memory products. MS-DOS (and the older versions of Microsoft windows it underlies) does not include support for USB (nor for USB booting). However there are some amateur projects to include USB support in DOS.
Some computers have the ability to boot up from keydrives, but that capability must be supported in the computer's BIOS.
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