Boot Basics

• When a computer boots up, several actions take place in succession. First, the system BIOS starts and scans through connected hardware devices in a prescribed order. When the BIOS finds a device with a valid boot sector, it uses that device to boot the system. Finally, it runs the software stored on the device. Bootable USB drives must pass each of these three phases without error by being listed at the top of the BIOS device list, having a properly written boot sector, and containing bootable software.
  
  

No Boot Sector

• A USB disk must contain a boot sector in order to work correctly at start-up. The boot sector is a standard piece of information read by the system's BIOS. Without it, the BIOS will skip over the USB drive as a non-bootable device and boot from a different piece of hardware. Several free software utilities can write boot sectors to USB drives, usually with specific software functions in mind. UNetbootin, for instance, creates boot sectors for Linux packages, while MKBT works for more general purposes.
  
  

Non-Bootable Software

• Even with a properly written boot sector, the USB drive won't boot properly unless it contains bootable software. Software may remain unbootable if it requires an operating system to run correctly, or has an incompatibility with the system's hardware. Most programs are not directly bootable, and have operating system requirements, but some specialized utilities run at start-up, often programs designed to repair or change the system, such OS install programs.
  
  

Incorrect BIOS Configuration

• The BIOS software keeps a list of bootable hardware devices, and will boot from the first available device in the list. In order to ensure it selects the USB drive, you must change the boot order to give priority to the USB ports. The steps required to enter the BIOS menu vary by motherboard manufacturer, but generally require you to press the correct function key during start-up. Look for a key labeled "BIOS" or "Setup" on the start-up screen. Go to the "Boot" or "Boot Order" section to change the device priority.
  
  

Incompatible Systems

• Some older motherboards don't list the USB ports in the BIOS menu, and don't support booting from USB at all. These platforms will not work with bootable USB devices even if they meet all three boot parameters. You may need to transfer the desired software to a CD or floppy disc to work with these systems.