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USB Flash Drives

USB flash memory drives are known by many names, both generic and trademarked. These devices are sometimes called keychain drives, watch-fob drives, key drives, pen drives, pocket drives, USB sticks, memory sticks, flash drives, USB mass storage drives, and so on. To the displeasure of the manufacturers, trade names for these devices are often used generically, including ThumbDrive (Tech 2000), JumpDrive (Lexar), and Gizmo! (Crucial Technology). Whatever you call them, they've become ubiquitous.

Figure 9-5 shows a collection of typical USB flash drives, with a Swiss Army Knife (not one of the models with a flash memory drive incorporated) shown for scale. The flash drives on the left are inexpensive and reliable Crucial Gizmo! units. Those on the right are a Kingston DataTraveller II (top) and the premium DataTraveller Elite.

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Figure 9-5: Typical USB flash drives

USB flash drives are conceptually simple. They comprise a physical housing that contains a USB interface connector, some flash memory, and the circuitry needed to interface the memory to the USB connector. Some drives include additional circuitry, such as support for hardware-based encryption or an LCD display. The drive plugs into and is powered by a USB port, and the flash memory is nonvolatile. Data written to the USB flash drive is retained permanently until it is manually deleted or overwritten. USB flash drives are most commonly used for backup and to "sneakernet" data between unconnected machines.

Here are the important characteristics of USB flash drives:


Current-generation USB 2.0 flash drives are available at prices ranging from $10 or so for low-capacity no-name units to $300 or more for the largest capacity name-brand drives. In general, within a particular manufacturer's line, price is closely related to capacity and speed, but there are very wide variations between manufacturers. A fast, namebrand 256 MB model, for example, may sell for about the same price as a slower, no-name 512 MB model.


First-generation USB flash drives used the USB 1.1 interface, which limited the effective data transfer rate to about 1 MB/s, too slow to be useful for transferring large amounts of data. Most current USB flash drives use the Hi-Speed USB 2.0 interface, which offers a theoretical data transfer rate of 60 MB/s, and real-world data transfer rates that may approach 25 MB/s under ideal conditions.


Current USB 2.0 flash drives are available with capacities ranging from 128 MB to 16 GB. First-generation USB 1.1 drives, some of which are still available at fire sale prices, typically have capacities from 16 MB to 128 MB.

Read and write performance

The read and write speeds of USB flash drives are quantified using the CD-ROM X-factor, where 150 KB/s (153,600 bytes/s) is defined as 1X. As is common practice for hard drives, USB flash drives define a kilobyte as 1,000 bytes and a megabyte as 1,000,000 bytes, and consider 1X speed to be 150,000 bytes/s rather than 153,600. For example, a USB flash drive that reads at 15,000,000 bytes/s is described as having 100X read speed.

Read performance transferring data from the drive to the host is affected by many factors, including the speed of the flash memory and interface circuitry used by the drive, the number and size of files being read, and the operating system being used. Every USB flash drive we tested reads a few large files faster than it reads many smaller files that total the same number of bytes, sometimes much faster. Absolute read performance differs dramatically from model to model, with less expensive models typically offering half or less the speed of premium models.

Write performance transferring data from the host to the drive is generally lower than read performance. On slower drives, write speed may be 90% of read speed, while on fast drives, write speed is often not much more than 50% of read speed. (Of course, read speed on fast drives may be three or four times faster than on slower drives.)

Encryption and password protection

A basic flash drive allows data to be written and read, but provides no other functions. More expensive models may include built-in encryption and/or password or biometric protection, provided by a dedicated chip on the drive and accessed by a driver that must be installed on any PC that is to read or write data on that drive. Frankly, we don't trust the encryption functions of such drives. We worry that our data will somehow become locked up and inaccessible. Fortunately, such drives function as standard USB flash drives if you simply don't install the encryption drivers. If we're concerned about the security of data that we've stored on a USB flash drive, we use standard encryption utilities to protect it. We recommend that you do the same.

U3 Smart USB Drives

U3 Smart USB Drives began shipping in October 2005. Although U3 drives provide standard USB flash drive storage functions, they are more than just storage devices. They allow you to carry your complete working environment with you, including data, programs, and personal preference settings. When you boot a system with a U3 Smart USB Drive installed, that system loads the data, programs, and settings stored on the drive. When you shut down the system, the system automatically synchronizes changes to the flash drive. The major limitation of U3 flash drives is their limited software support. They can be used only with Windows 2000 or Windows XP and, for example, support Outlook but not other popular email clients such as Thunderbird. For more information, see

More about External Storage Devices

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