How to boot Backtrack from usb?

It is possible. If you have it installed on the Flash Drive, here’s how to boot from it:

1. Change the BIOS boot order so the USB device option is listed first. The BIOS is rarely setup this way by default.

   If the USB port is not first in the boot order, your PC will start “normally” (I.e. boot from your hard drive) without even looking at any boot information that might be on your USB device.

   Note: After setting your USB device as the first boot device, your computer will check it for boot information each time your PC starts. Leaving your computer configured this way shouldn’t cause problems unless you plan on leaving the USB device attached all the time.

2. Attach the USB device to your computer via any available USB port.

   How do you know if a USB device is bootable? The easiest way to find out if a flash drive or external hard drive is bootable is to plug it in and follow the rest of these instructions.

   Note: Creating a bootable flash drive or configuring an external hard drive as bootable is a task in itself. Follow any instructions given with the data that should be bootable.

3.Restart your computer.

4.Watch for a Press any key to boot from external device… message.

  On some bootable devices, you may be prompted with a message to press a key before the computer will boot to the flash drive or other USB device.

  If you do nothing, your computer will check for boot information on the next boot device in the list in BIOS (see Step 1) which will probably be your hard drive.

  Note: Most of the time when trying to boot to a USB device there is no key-press prompt. The boot process usually starts immediately.

5.Your computer should now boot from the USB flash drive or USB based external hard drive.

  Note: What happens now depends on what the bootable USB device was intended for. If you're booting to an old version of MS-DOS on a flash drive, MS-DOS will load. If you're booting to the DSL version of Linux, it will start. You get the idea.

  If you tried the above steps but your computer did not boot from the USB device, check out some of the tips below.

Tips:

1. Recheck the boot order in BIOS (Step 1). The number one reason a bootable flash drive or other USB device won’t boot is because BIOS is not configured to check the USB port first.
2. Didn’t find a “USB Device” boot order listing in BIOS? If your computer was manufactured around 2001 or before, it may not have this ability. If your computer is newer, check for some other ways that the USB option might be worded. In some BIOS versions, it’s called “Removable Devices” or “External Devices”.
3. Switch to another USB port. The BIOS on some motherboards only check the first few USB ports. Switch to another USB port and restart your computer.
4. Copy the files to the USB device again. If you created the bootable flash drive or external hard drive yourself, which you probably did, repeat whatever steps you took again. You may have made a mistake before or missed an important file.

http://pcsupport.about.com/od/tipstricks/ht/bootusbflash.htm

And here’s how to install it, if you haven’t installed yet:

http://www.pendrivelinux.com/usb-backtrack-linux-installation/