dd command to create bootable iso image of hard disk

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http://stackoverflow.com – Is it possible to use dd command to create a bootable ISO image from hardisk ? I use below command: dd if=/dev/sda of=/mnt/sdb/my.iso I am able to create ISO, but when i try to boot i get error, Operating System not found /dev/sda got 4 partition and in partition 1, grub is installed, filesystem is ext3 Thanks, Saqlain. (HowTos)