Live USB

USB drive with a full bootable operating system

A live USB is a portable USB attached external data storage device containing a full operating system that can be booted from. The term is reminiscent of USB flash drives but may encompass an external hard disk drive or solid state drive, though they may be referred to as "live HDD" and "live SSD" respectively. They are the evolutionary next step after live CDs, but with the added benefit of writable storage, allowing customizations to the booted operating system. Live USBs can be used in embedded systems for system administration, data recovery, or test driving, and can persistently save settings and install software packages on the USB device. Many operating systems including Mac OS 9, [[OS X, Windows XP Embedded and a large portion of Linux and BSD distributions can run from a USB flash drive, and Windows 8 Enterprise has a feature titled Windows To Go for a similar purpose.

To repair a computer with booting issues, technicians often use lightweight operating systems on bootable media and a command line interface. The development of the first live CDs with graphical user interface made it feasible for non technicians to repair malfunctioning computers. Most Live CDs are Linux based, and in addition to repairing computers, these would occasionally be used in their own right as operating systems. Personal computers introduced USB booting in the early 2000s, with the Macintosh computers introducing the functionality in 1999 beginning with the Power Mac G4 with AGP graphics and the slot loading iMac G3 models. Intel based Macs carried this functionality over with booting macOS from USB. Specialized USB based booting was proposed by IBM in 2004 with Reincarnating PCs with Portable SoulPads and Boot Linux from a FireWire device.

Live USBs share many of the benefits and limitations of live CDs, and also incorporate their own.

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 their 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, thereby avoiding the rotational latency and seek time 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 older computers that lack support for USB 2.0 or newer can be very slow.

LiveUSB OSes like Ubuntu Linux apply all filesystem writes to a casper filesystem overlay (casper rw) that, once full or out of flash drive space, becomes unusable and the OS ceases to boot. 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. Some 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 do this only in EFI mode. When the firmware switches to "legacy" BIOS mode, it no longer recognizes USB drives. Non Macintosh systems, notably Windows and Linux, may not be typically booted in EFI mode and thus USB booting may be limited to supported hardware and software combinations that can easily be booted via EFI. However, programs like Mac Linux USB Loader can alleviate the difficulties of 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 locked SD 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; .

One alternative to the live operating system solution would be a full installation; that is, a traditional installation, but without swap partitions. This option has the advantage of being efficient for the software, as it eliminates the need for the device to retain – separate from the persistent file – the operating system’s installer software. However, full installation is not without disadvantages; due to the additional write cycles that it requires, 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 initiates a system powerdown, which triggers the actual writes to the device. The trade off is greater risk of lost work, in the case of an abnormal abort. Beyond these, another factor to consider is that, if the transfer speed of the storage device is poor, then performance can be reduced to a rate more typical of legacy computers – even for machines with modern components. This issue can usually be overcome by installing to a USB hard drive, as they generally perform better than flash drives, regardless of the connector.

Although many live USBs rely on booting an open source operating system such as Linux, it is possible to create live USBs for Microsoft Windows by using Diskpart or WinToUSB.