Why dd is not a reliable command to write bootable .iso files to USB thumb drive?

Solution 1:

With Ubuntu 12.04 it is possible to dd the .iso file to a disk to create a bootable disk. It only works with some isos (including Ubuntu 12.04, but not earlier Ubuntu versions) because booting from CD and booting from a disk are handled differently on BIOS based systems.

Among other things, for a hard drive to be bootable it must contain boot code in the first sector, for a CD to be bootable it must contain boot code "at sector 11 (17 decimal) in the last session on the CD" according to the El Torito specification. Since the boot code, at least the initial portion loaded by the BIOS, is stored in different places for CDs and hard drives, a disk image can have boot code in both places so that the bootloader is loaded if the image is booted as a hard drive or as a CD.

Most iso files are only designed to be bootable as CDs, and so they contain no boot code in the first sector of the image and thus a hard drive containing such an image won't have boot code in its first sector and won't be bootable.