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 lacking USB 2.0 support can be very slow.
LiveUSB OSes like Ubuntu GNU/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.
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 GNU/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 GNU/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 GNU/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 GNU/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
(Actual use of a CD or DVD will 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 re-booted.)
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.
Note that in the following instructions for using the diskpart program from the cmd interface, you should replace the '1' in 'select disk = 1' with the relevant number as deduced from the 'list disk' command. Make sure you do choose the correct disk, because the 'clean' command will wipe all data from that disk.But don't worry you can get your data back again by using system restore functions of windows OS Please type the following snippet into your command prompt. However this function can work only on windows7 & windows8 only not on XP. Still it is one of the best method.
select disk = 1
create partition primary
select partition 1
format fs=ntfs quick
(select your local drive where you have store the windows os)
cd (Give your file location)
bootsect.exe/nt60 (your pendrive id)
By Abhishek Tiwari
The second type of live USB is closely related to a traditional operating system hard drive install with minor modifications like the elimination of swap partitions and files.
Updating applications or the whole thing is as easy as the parent distribution used to create it.
Full system encryption possible.
Easier to customize with the user's preferred Window Manager and applications.
Base install usually starts at approximately 200 MB (although some can be as little as 40 MB) and grows as the user adds applications.
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.