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According to Wikipedia, an ISO file is (emphasis mine):

ISO disc images are uncompressed and do not use a particular container format; they are a sector-by-sector copy of the data on an optical disc, stored inside a binary file. ISO images are expected to contain the binary image of an optical media file system (usually ISO 9660 and its extensions or UDF), including the data in its files in binary format, copied exactly as they were stored on the disc. The data inside the ISO image will be structured according to the file system that was used on the optical disc from which it was created.

When I use dd or some other low-level utility to write the raw ISO bytes to a USB stick I can't boot from the USB stick. Why is that? The raw filesystem structures are present on the USB stick. Sure, UDF or other filesystems are not ideal for usb but they should work, right?

I have tried this with the newer images of Windows 10 and Ubuntu 16.

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    It sounds like you're looking for this: superuser.com/questions/407324/…
    – James Vickery
    Jun 3, 2016 at 17:11
  • Although this does not answer your question, but if the end goal is to create a USB that can boot your ISO - you could use YUMI (personal favourite - no attachment to the creator) to create a USB that boot multiple ISO or other things as needed. I have created custom ISO and used Yumi to boot them and they generally works. - Beyond that - the above comment is a good proposed answer.
    – Darius
    Jun 7, 2016 at 2:08
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    @James: I'm not looking for a how, I'm looking for a why.
    – Sebazzz
    Jun 7, 2016 at 7:16

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The data will write out just fine, but the boot information in the first sector is specific to CD/DVD drives, and won't work on USB drives. Everything else - down to (as you noted) the raw filesystem structure - is effectively media-independent, but the boot information is not.

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    Thanks - I understand that, but I want to know the reason why it doesn't work. Is, for example, the BIOS hardcoded to only accept such boot information from CD drivers?
    – Sebazzz
    Jun 4, 2016 at 8:20
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This depends on special arrangements that could be done for particular ISO image by its creator.

For example, the current 'official' method to create a bootable USB stick out of Fedora ISO image is exactly dd if=IsoImageFile.iso of=/dev/sdX

Source

Indeed, if you look into the first 512 bytes of any F23's LiveCD image you will see there a conventional boot loader.

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