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I've long known that you cannot simply put a disk image on a CD or USB flash drive and expect it to boot, but what characteristics make an image bootable?

I ask this because one of my computers crashed and I was trying to make a bootable version of DBAN on my Ubuntu computer. None of the Linux GUI programs used for writing an image to a USB flash drive have worked for me, and I tried to use the dd command to do this.

It wrote to the flash drive successfully, and even retitled the USB flash drive in the filemanager DBAN and put the files in such a way that it seemed to do it the right way. However, it wasn't bootable from any computer. But I tried using Brasero to write the image to the disk and it worked as it always does.

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The presence of a Boot Sector that bootstraps a system image of some kind. In the old days, to make a floppy MS-DOS bootable, you had to format it to be bootable (which created a MBR on the 1st and 63rd sectors of the disk, and marked the partition as Active) and copy some system files to it. Some filesystems and BIOSes require special flags to tell the BIOS whether a system is expected to implement a boot sector of some kind.

When the system attempts to boot, the BIOS/Firmware selects the bootsector from the boot device, places it in RAM, and then reads it to determine where the bootloader is on disk. Then it executes the boot loader.

In the case of a CD, the ISO-9660 specification defines a Primary Volume Descriptor specifying a TypeCode of '0' and an El-Torito Boot Record at address 0x11, which in turn indicates a Boot Catalog structure. The Firmware loads a boot instruction from the Catalog, indicating the location of the boot loader.

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