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Some softwares offer both a .iso file and a .img file. The .iso is supposed to be burn to a CD/DVD, while the .img file is to be written directly on a USB disk. But other softwares only provide a .iso file. Then how to create a bootable USB from this .iso file?

Please note that I'm not asking about tools to do this. I'd like to know what's the difference between the .iso and the .img files, and why or why not writing the .iso file directly to a USB disk works. If directly writing an .iso file to a USB disk doesn't work, how can we create a flashable .img file from the .iso file?

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One of the reasons for providing different formats is for convenience and semantics. ISO files are typically images of discs while img files are typically images of a hard disk or partition, historically img file was for floppy disk's. IMG files if used with the right software can clone HDD's even down to the partition layout and whether or not a partition is bootable etc.

As for the difference between installing from usb or iso, its virtually the same. You could extract the files from the ISO, place them on the usb, setup the syslinux config file and all the necessary partitions such as the boot partition as well as the master boot record if booting from legacy bios or setting the boot partition as efi or esp depending on os being installed, windows calls it an esp partition if I recall. So you can manually create a bootable usb. However sotware like rufus streamlines the process, as well as providing an img file since it already contains an exact copy of the layout.

  • So how to setup the syslinux config? How do you know which kernel and ramdisk to use? Is it possible to jump to the boot code in the ISO format? – Cyker Jan 6 '17 at 5:30
  • If you do a search you can find sources and examples for configuring syslinux config especially for ubuntu. The kernel is selected automatically unless you want a specific kernel to load. Not sure what you mean jump to the boot code in the ISO format. What you can do is extract the boot image from the ISO if it is a hybrid image. – Frostalf Jan 9 '17 at 10:29
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You can create your bootable usb drive from .iso with

(run as root)

dd if=[name of iso] of=/dev/sdb bs=1M
sync
eject /dev/sdb

Just pay attention of the /dev/sdb corresponding to your usb drive.

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    -1, An .iso file (unless it's a hybrid ISO/IMG) does not have a MBR boot program that is expected in the first sector. – sawdust Jan 5 '17 at 8:44

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