This is a how/to on booting from a usb flash or hard drive. This page assumes you know how to navigate your computers bios options.
1. Make sure the drive is boot-able. You can make a linux boot image using the USB Startup Disk Creator utility in the newer Ubuntu releases or with the dd command in a termnial. Note: Not all disk images support USB booting
2. After you make your usb drive boot-able, and installed a disk image to it, restart your computer and press the button to choose the disk to boot into when prompted, the exact button is different from system to system.
3. If your computer does not have the option to choose the boot disk from the BIOS screen, press the button to enter BIOS setup.
4. Go to the boot order settings in the BIOS options and you should see an option to set the boot priority of the HDD group. (a boot-able USB drive is recognized as an HDD)
5. Select that option and choose the one that has “USB” in the name as the first disk.
6. Save and exit, and the computer should boot into your usb drive.