Live USB

A live USB is a USB flash drive or external hard disk drive containing a full operating system that can be booted. 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 , , 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.
Background.
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 GNU/Linux from a FireWire device".
Benefits and limitations.
Live USBs share many of the benefits and limitations of live CDs, and also incorporate their own.
Setup.
Various applications exist to create live USBs; examples include Rufus, Fedora Live USB Creator, and UNetbootin. A few Linux distributions and live CDs have ready-made scripts which perform the steps below automatically. In addition, on Knoppix and Ubuntu extra applications can be installed, and a persistent file system can be used to store changes. A base install ranges between as little as 16 MiB (Tiny Core Linux) to a large DVD-sized install (4 gigabytes).
To set up a live USB system for commodity PC hardware, the following steps must be taken:
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 or flash media to preserve any added configurations and security updates. This can be easier than recreating the USB system but may be moot since many live USB tools are simple to use.
Full installation.
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 system’s 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 poor; 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.
Microsoft Windows.
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.