isohybrid

http://gihyo.jp/admin/clip/01/ubuntu-topics/201106/24

「Isohybrid」とは，「ISO9660イメージとして機能しつつ，かつ，そのままdd等でUSBメモリに書き込むことで，ブータブルUSBメモリを作成できる」（＝CD用イメージでありながら，同時にUSBメモリ用イメージでもある）というものです。

https://fedorahosted.org/fedora-engineering-services/ticket/15
元はPerlスクリプトだったのね。
DOS類似のパーティションテーブルを追加しているのかな。
2008年からあった？
http://www.zytor.com/pipermail/syslinux/2008-September/010812.html

* ISOLINUX: support generating images which can be either
a CD-ROM or a hard disk (USB disk, etc.) See
doc/isolinux.txt for more information.

++++ HYBRID CD-ROM/HARD DISK MODE ++++

Starting in version 3.72, ISOLINUX supports a "hybrid mode" which can
be booted from either CD-ROM or from a device which BIOS considers a
hard disk or ZIP disk, e.g. a USB key or similar.

To enable this mode, the .iso image should be postprocessed with the
"isohybrid" script from the utils directory:

isohybrid filename.iso

This script creates the necessary additional information to be able to
boot in hybrid mode. It also pads out the image to an even multiple
of 1 MB.

This image can then be copied using any raw disk writing tool (on Unix
systems, typically "dd" or "cat") to a USB disk, or written to a
CD-ROM using standard CD burning tools.

The ISO 9660 filesystem is encapsulated in a partition (which starts
at offset zero, which may confuse some systems.) This makes it
possible for the operating system, once booted, to use the remainder
of the device for persistent storage by creating a second partition.

ほんとだ。