iso image - Why is an iso file needed to burn into a USB

I wonder why an iso file is needed to be burned into a USB in general?

For example, I have downloaded the installation iso file of Ubuntu 12.04. I want to install from my USB, so I copy the iso file to my USB. But then I cannot install from my USB.

I am told that the iso file must be burned into the USB first. I don't understand What "burn" means actually? why should the iso be burned?

Thanks and regards!

Answer

• The the BIOS tries to boot from a device (flash drive, hard disk, optical drive, floppy disk, etc.), it searches for a boot sector.

  
  
  

  From Boot sector - Wikipedia:

  
  
  

  
  
  

  A boot sector or boot block is a region of a hard disk, floppy disk, optical disc, or other data storage device that contains machine code to be loaded into random-access memory (RAM) by a computer system's built-in firmware. The purpose of a boot sector is to allow the boot process of a computer to load a program (usually, but not necessarily, an operating system) stored on the same storage device. The location and size of the boot sector (perhaps corresponding to a logical disk sector) is specified by the design of the computing platform.

  
  

  
  
  

  Just copying an .iso file to the flash drive won't create a boot sector, so it will not work. Likewise, you cannot simply copy all files from on hard disk to another and expect the installed OS to work. The files are there, the boot sector isn't.




• An ISO image is an archive. The files inside the ISO file have to get extracted or they're will be unusable.

  
  
  

  Likewise, you cannot archive the entire content of your hard disk in a .zip file and keep using it.




• The term burning is utterly misused when talking about flash drives.

  
  
  

  From ISO image - Wikipedia:

  
  
  
  

  
  

  An ISO image is [...] an archive file [...], composed of the data contents of every written sector of an optical disc, including the optical disc file system.

  
  

  
  
  

  That means it contains all necessary files and boot sector information to make your OS boot. Traditionally, burning means writing this information to an optical disk. The laser actually burns the data on a disc's layer. When talking about flash drives, burning just means writing the data to the flash drive.