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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!

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It's not "Burning" (with lasers). Also, remember... ISO files are pushed (not burned) on original DVDs etc. But, we burn at home (a cheap solution). –  SS-3.1415926535897932384626433 May 26 '12 at 23:52
    
@Sachin, pressed. –  Synetech May 27 '12 at 0:04
    
@Tim ,technically you don’t. You should be able to copy files to and from the drive so long as the boot-record is already present. In other words, after you write it once, you should be able to subsequently use the drive as normal. (Of course it depends on the format the the drive uses; if it emulates a CDVD, then you might not, but if it emulates a hard-drive, you should be able to.) –  Synetech May 27 '12 at 0:06
    
@Synetech Oh, sorry... Pressed. :) –  SS-3.1415926535897932384626433 May 27 '12 at 0:17
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@Sachin, no worries. It’s really interesting that they can make the molds so small and fine that they can literally press them into the hot glass to make the microscopic pits. It always blows me away that we can make things so microscopically small. –  Synetech May 27 '12 at 0:20

3 Answers 3

up vote 6 down vote accepted
  • 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.

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Thanks! I am still having questions about ISO file format. (1) Does the whole content on every CD exist as an ISO archive file? Or are there many CDs whole entire contents are not ISO archives? (2) If I have an ISO file on my hard drive, can I simply copy this ISO file to a blank CD, so that this CD can be used later for accessing the files in the ISO archive? –  Tim Jun 2 '12 at 18:07
    
(1) The content of a CD isn't an ISO image. The ISO is an image of the content, that is, a bit-by-bit copy of everything on the CD. (2) With an archiving tool capable of handling ISO images, you can access the files inside an ISO image just fine. However, this will generally not work for booting from the CD, installing software, playing games, etc. For the latter two purposes, you'll have to burn or mount the image. –  Dennis Jun 2 '12 at 18:21

If you read instructions that told you to specifically "burn" an ISO of Ubuntu onto a USB flash drive, it was a misuse of the word... in so much that the use of "burn" takes some liberties and makes some assumptions.

In relation to ISO files, to "burn" one means to use a program that makes a disc from the ISO file. This is not the same as copying or writing the ISO file onto a disc... where if you then examined the disc with Explorer, it would show the actual file with the extension .ISO on the disc. To burn a disc from an ISO file means to use software that will take the disc image, and turn it back into an actual disc. In a way, an ISO file is similar to a .zip file. It is a package that contains everything the original disc contained. So to "burn" a disc from an ISO file means to take that container, and open it back up onto a disc.

Now, the act of "burning" the ISO file to a USB flash drive, means to copy the contents within the ISO file onto the drive, not copy the actual ISO file onto the drive. It also takes into account that you will have to make the USB drive bootable, as if it was a bootable CD or DVD. Just opening the ISO file and copying the files over isn't enough.

As you can see here, the instructions for installing Ubuntu from a USB flash drive are a little more complicated than just copying the ISO file over to the thumbstick.

From Windows

The Oneiric ISO file no longer includes a file called usb-creator.exe in the CD image. You need to download Linux Live Usb Creator.

Once you have usb-creator.exe, run it and follow the same steps as described for Linux (point it at your .iso file or your Ubuntu CD-ROM, point it at your USB flash drive, make sure you have the right device selected, then "Make Startup Disk").

Notes

Instead of usb-creator.exe you can use Unetbootin to create a bootable USB flash drive. http://unetbootin.sourceforge.net/

You won't be able to select the USB flash drive if it wasn't formatted in a way that Windows can see it. You may have to format it using Windows Explorer in order for it to show up in a creator tool.

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Think of an .iso file like you would a .zip file. Maybe that would help.

However unlike a .zip file, the .iso is a complete copy, or image of an entire drive. The .iso file is also something that can be "mounted" just like any other physical/logical drive can too. But the .iso is still not a real device.

Of course, you can modify the contents of an .iso without actually installing, re-imaging, etc. But that takes a whole lot more steps than a simple .zip file would. And modifying the contents of a .iso is also not a built in function of Windows either (at least not yet).

So in essence, the .iso file is a virtual drive that has to be made real through a process called "burning" in order for a real system to use the information within - like booting.

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Thanks! "unlike a .zip file, the .iso is a complete copy, or image of an entire drive", do you mean that .iso does not compress the files, while .zip does? –  Tim Jun 4 '12 at 12:30

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