A live USB is a USB flash drive or a USB external hard disk drive which contains a full operating system that can be booted. Live USBs are closely related to live CDs, but can sometimes persistently save settings, and can permanently install software packages back onto the USB device. Like live CDs, live USBs can be used in embedded systems for
Since 1999, Apple Macintosh computers (beginning with the Power Mac G4 with AGP graphics and the slot-loading iMac G3 models) have been able to boot from USB. Intel-based Macs support booting Mac OS X from USB.
In contrast to live CDs, the data contained on the booting device can be changed and additional data stored on the same device. A user can carry his or her preferred operating system, applications, configuration, and personal files with them, making it easy to share a single system between multiple users.
Live USBs provide the additional benefit of enhanced privacy because users can easily carry the USB device with them or store it in a secure location (e.g. a safe), reducing the opportunities for others to access their data. On the other hand, a USB device is easily lost or stolen, so data encryption and backup is even more important than with a typical desktop system.
The absence of moving parts in USB flash devices allows true random access avoiding the rotational latency and seek time (see mechanical latency) of hard drives or optical media, meaning small programs will start faster from a USB flash drive than from a local hard disk or live CD. However, as USB devices typically achieve lower data transfer rates than internal hard drives, booting from a computer that lacks USB 2.0 support can be very slow.
USB controllers on add-in cards (e.g., ISA, PCI, and PCI-E) are almost never capable of being booted from, so systems that do not have native USB controllers in their chipset (e.g., such as older ones before USB) likely will be unable to boot from USB even when USB is enabled via such an add-in card.
Some computers, particularly older ones, may not have a BIOS that supports USB booting. Many which do support USB booting may still be unable to boot the device in question. In these cases a computer can often be "redirected" to boot from a USB device through use of an initial bootable CD or floppy disk.
Intel-based Macintosh computers have limitations when booting from USB devices – while the Extensible Firmware Interface (EFI) firmware can recognize and boot from USB drives, it can only do this in EFI mode. When the firmware switches to "legacy" BIOS mode, it no longer recognizes USB drives. Non-OS X systems may not be typically booted in EFI mode, notably Windows and Linux, and thus USB booting may be limited to supported hardware and software combinations, which can easily be booted via EFI, however, programs like Mac Linux USB Loader can alleviate the task of booting a Linux-live USB on a Mac. This limitation could be fixed by either changing the Apple firmware to include a USB driver in BIOS mode, or changing the operating systems to remove the dependency on the BIOS.
Due to the additional write cycles that occur on a full-blown installation, the life of the flash drive may be slightly reduced. This doesn't apply to systems particularly designed for live systems which keep all changes in RAM until the user logs off. A write-lockedSD card (known as a Live SD, the solid-state counterpart to a Live CD) in a USB flash card reader adapter is an effective way to avoid any duty cycles on the flash medium from writes and circumvent this problem. The SD card as a WORM device has an essentially unlimited life. An OS such as Linux can then run from the live USB/SD card and use conventional media for writing, such as magnetic disks, to preserve system changes; see persistence (computer science).
To set up a live USB system for commodity PC hardware, the following steps need to be done:
A USB flash drive needs to be connected to the system, and be detected by it
One or more partitions may need to be created on the USB flash drive
The "bootable" flag must be set on the primary partition on the USB flash drive
An MBR must be written to the primary partition of the USB flash drive
The partition must be formatted (most often in FAT32 format, but other file systems can be used too)
A bootloader must be installed to the partition (most often using syslinux when installing a Linux system)
A bootloader configuration file (if used) must be written
The necessary files of the operating system and default applications must be copied to the USB flash drive
Language and keyboard files (if used) must be written to the USB flash drive
USB support in the BIOS' boot menu (although there are ways to get around this; actual use of a CD or DVD can allow the user to choose if the medium can later be written to. Write Once Read Many discs allow certainty that the live system will be clean the next time it is rebooted.)
Knoppix live CDs have a utility that, on boot, allows users to declare their intent to write the operating system's file structures either temporarily, to a RAM disk, or permanently, on disk and flash media to preserve any added configurations and security updates. This can be easier than re-creating the USB system but may be moot since many current (circa 2010) live USB tools are simple to use.
An alternative to a live solution is a traditional operating system installation with the elimination of swap partitions. This installation has the advantage of being efficient for the software, as a live installation would still contain software removed from the persistent file due to the operating systems installer still being included with the media. However, a full installation is not without disadvantages; due to the additional write cycles that occur on a full installation, the life of the flash drive may be slightly reduced. To mitigate this, some live systems are designed to store changes in RAM until the user powers down the system, which then writes such changes. Another factor is if the speed of the storage device is destitute; performance can be comparable to legacy computers even on machines with modern parts if the flash drive transfers such speeds. One way to solve this is to use a USB hard drive, as they generally give better performance than flash drives regardless of the connector.