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Following on from this question:

Can I burn a CD ISO to DVD?

How/why does this work?

Is an ISO image a physical media neutral format?

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up vote 5 down vote accepted

An ISO is just a file describing data on a volume. Many of them tend to be the same size and type of data as you would find on CDs or DVDs, since that's where they're more useful--but the size of an ISO is completely arbitrary in reality.

You could "burn" the data from an ISO onto any volume large enough to hold it, be it flash drive, hard drive, DVD, or Blu-Ray; we just use the terminology of "burn" since that's what you have to do for optical media. ISOs can also be mounted as virtual volumes without burning them to anything--the Mac-only equivalent is the disk image, or DMG, and this is a very common method of distributing software.

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To back up what NReilingh said and address your supplementary question:

Yes an ISO image is media neutral. As long as the physical media has enough space you can write an iso to almost anything.

With the right software you can even read an ISO direct from the hard drive. See the Wikipedia page on virtual drives for more information

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I'm well aware of mounting ISO's and have been doing it without thought or question for years. But I was puzzled by section 2 on the Ubuntu download page: ubuntu.com/desktop/get-ubuntu/download which only refers to burning to a CD. I then had to google to find out if I could burn this to DVD thinking that perhaps the ISO image was only laid out for CD. In addition the thought had crossed my mind that ISO mounters did some clever remapping based on querying the ISO to ask for its format (CD or DVD) and thus presenting the correct device emulation to the OS. (cont...) – Kev Aug 21 '10 at 12:57
    
I also thought that the tools that prepare/write ISO's to USB sticks also did some clever remapping of tracks and sectors. It was all a bit mysterious to me, hence asking the question. – Kev Aug 21 '10 at 12:58

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