Question

32

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

rated 0 times [ 32] [ 0] / answers: 1 / hits: 27367 / 2 Years ago, sat, december 25, 2021, 4:47:10

One answer here indicates that Ubuntu .iso s are not expected to boot if copied with dd to a USB thumb drive.

Why do some Linux distributions have the option to directly write their bootable .iso file to a thumb drive with dd, but others (Ubuntu?) do not?

In Ubuntu I think it has to be converted to .img first. Is this true?

Is it for some architectural difference in .isos? Or is it due to any limitation of dd itself?

I don't know if it is off-topic here. I can move it to a more proper place if the community thinks so or suggests one. Some explanation would be appreciable.

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answered 2 Years ago curatekill · Accepted Answer

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.