[gentoo-user] Re: [OT] Making a bootable USB flash drive from ISO image

[Grant Edwards]

On 2014-10-02, Neil Bothwick <neil@××××××××××.uk> wrote:
> On Thu, 2 Oct 2014 21:11:03 +0000 (UTC), Grant Edwards wrote:
>
>> What I'm wondering is why all the blog posts, wiki pages, and HOWTOs
>> showing either the complicated command-line procedures or
>> dependency-heavy "USB creator" apps?  (Many of them quite recent.)
>
> There is no need for them.

>> Did this work because the Xubuntu people do something special when
>> creating the ISO image?  (If so, then they have my thanks!)
>
> They did something, but it's not particularly special. All they did was
> run isohybrid (from syslinux) on the ISO image. It uses unused space at
> the start of the DVD ISO specification that is normally left blank to
> hold a partition table so the image can be copied to a USB drive and
> appear as a partitioned disk.

There must be a little more to it than that.  Does it also install
along with a parition table some bootloader code that knows where/how
to boot the ISO boot image?  [After a little googling] It also
installs MBR bootloader code that knows how to find/load isolinux.bin:

http://www.syslinux.org/wiki/index.php/Isohybrid

Interestingly, it also created a second EFI boot partition which
exists inside the main parition.

  $ sudo fdisk -l /dev/sdd
  
  Disk /dev/sdd: 7.3 GiB, 7864320000 bytes, 15360000 sectors
  Units: sectors of 1 * 512 = 512 bytes
  Sector size (logical/physical): 512 bytes / 512 bytes
  I/O size (minimum/optimal): 512 bytes / 512 bytes
  Disklabel type: dos
  Disk identifier: 0x19fabbe1
  
  Device    Boot     Start       End Blocks  Id System
  /dev/sdd1 *            0   1904639 952320   0 Empty
  /dev/sdd2          14296     18967   2336  ef EFI (FAT-12/16/32)

I can mount the sdd2 VFAT partition, and there are a couple files
in there:

sdd2
└── efi
    └── boot
        ├── bootx64.efi
        └── grubx64.efi

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