Debian installer on a USB key with a writable file system.

bootable-flash.txt

Debian installer on a USB key with a writable file system.

I couldn't find a concise article about creating a Debian installer USB 
key with a writable file system, so here is my take.  This assumes you 
have an available Linux system.  Note that some old BIOSes might not 
happily boot USB drives created in this way.

* Install the packages syslinux dosfstools mbr
* Insert the thumb drive and find the device using dmesg 
* Install a Master Boot Record to the drive (install-mbr /dev/sdX)
* Create a DOS filesystem (mkdosfs /dev/sdX1)
* Put syslinux on the drive (syslinux /dev/sdX1)

Now mount the drive.

Add the initial ramdisk image and kernel binary (initrd.gz and vmlinuz) from 
http://http.us.debian.org/debian/dists/stable/main/installer-amd64/current/images/hd-media/

Add a Debian installation ISO.  This may be netinst, a CD, DVD, or BD.  
For a BD, you will need to repartition and reformat[1] the thumb 
drive for ext4fs or some other Linux filesystem.  This is because a 
FAT32 filesystem cannot handle files larger than 4 GB.  

Create a text file on the drive named "syslinux.cfg".  At a bare 
minimum, it should contain a single line:

default vmlinuz initrd=initrd.gz

Here's the new and "proper" way to do it:

DEFAULT BootUSB
LABEL BootUSB
KERNEL vmlinuz
APPEND initrd=initrd.gz 
TIMEOUT 1

Automating the installation is very helpful and a big reason for why 
you'd want a writable installer thumb drive in the first place.
This is the syslinux.cfg file you need to get that started:

DEFAULT BootUSB
LABEL BootUSB
KERNEL vmlinuz
APPEND initrd=initrd.gz auto file=/hd-media/preseed.cfg locale=en_US 
console-keymaps-at/keymap=us
TIMEOUT 1

Now you'll need a file "preseed.cfg" placed in the root of the thumb 
drive.  See http://www.debian.org/releases/stable/amd64/apb.html.en for 
more information.

Here's something basic:

d-i debian-installer/locale string en_US.UTF-8
d-i keymap select us
d-i keyboard-configuration/xkb-keymap select us
d-i clock-setup/utc boolean true
d-i time/zone string US/Pacific
d-i clock-setup/ntp boolean true
d-i passwd/user-default-groups string cdrom floppy sudo audio dip \
	video plugdev netdev scanner bluetooth vboxusers
d-i partman/mount_style select traditional
d-i mirror/country string US
d-i mirror/http/hostname string http.us.debian.org
d-i mirror/http/directory string /debian
d-i popularity-contest/participate boolean false
d-i apt-setup/use_mirror boolean true
d-i pkgsel/include string build-essential devscripts flex bison gdb \
	autoconf automake cmake pkg-config libtool intltool-debian


Footnotes
=========
[1]
If you want to put a bluray ISO on your thumb drive, you'll need to 
repartition and reformat the drive for ext4fs or some other Linux 
filesystem.  Run "sudo fdisk /dev/sdX", the delete all partitions.  Make 
a new primary partition taking up the whole drive.  Mark it type 83 for 
Linux.  Mark it bootable.  Type 'w' to write the partition info, then 
quit.  Back to the command line, type "mkfs.ext4 /dev/sdX1".  This 
formats the partition.  Mount the drive.  Type "extlinux --install 
/flash" or wherever you mounted the drive.  The extlinux utility comes 
with the syslinux package.

Sources
=======
http://hyper.to/blog/link/debian-installer-on-a-usb-key/
https://www.debian.org/releases/stable/amd64/ch04s03.html.en