Installing Debian on a USB stick

1.  Creating a USB version of a live CD — the boring option
With the current 'hybrid' Debian iso's it couldn't be easier.
THIS WILL WIPE YOUR USB STICK

 — START HERE —

a. Download the iso
Using jigdo is a good option. Downloading a business-card/netinstall iso is another, if you'll have a working internet connection available.

b. Plug in and mount your USB device and  find out the device name of your USB drive.
If it's mounted you can use
#df -h

rootfs                                                   93G   36G   54G  40% /
udev                                                    3.9G     0  3.9G   0% /dev
tmpfs                                                   801M  980K  800M   1% /run
/dev/disk/by-uuid/..-10a350f85687   93G   36G   54G  40% /
tmpfs                                                   5.0M     0  5.0M   0% /run/lock
tmpfs                                                   1.6G   52K  1.6G   1% /tmp
tmpfs                                                   1.6G  816K  1.6G   1% /run/shm
/dev/sda6                                               745G  183G  525G  26% /home
/dev/sdc1                                               2.0G  434M  1.5G  23% /media/XP-KOMKU

or

mount

../dev/sdc1 on /media/XP-KOMKU type vfat (rw,nosuid,nodev,relatime,uid=1000,gid=1000,fmask=0022,dmask=0077,codepage=cp437,iocharset=utf8,shortname=mixed,showexec,utf8,flush,errors=remount-ro,uhelper=udisks)

to list mounted devices. In my case it's an old 2 G usb stick I used to create a Windows XP installation USB device on.

If you prefer a gui tool, start palimpsest (called Disk Utility in gnome)
It will be something akin to sdb1 or sdc1 etc. That means the device name is /dev/sdb or /dev/sdc, respectively. In our case, it's sdc.

 

c. Unmount but don't detach the device
You don't want anything else writing to it.
 

# umount /dev/sdc1
# df -h
Filesystem                                              Size  Used Avail Use% Mounted on
rootfs                                                   93G   36G   54G  40% /
udev                                                    3.9G     0  3.9G   0% /dev
tmpfs                                                   801M  976K  800M   1% /run
/dev/disk/by-uuid/..-10a350f85687   93G   36G   54G  40% /
tmpfs                                                   5.0M     0  5.0M   0% /run/lock
tmpfs                                                   1.6G   60K  1.6G   1% /tmp
tmpfs                                                   1.6G  820K  1.6G   1% /run/shm
/dev/sda6                                               745G  183G  525G  26% /home

d. Become root and use cat to write the iso to the USB device

sudo su

And write (assuming that the device name is sdc and you are using the businesscard iso)

cat debian-6.0.3-amd64-businesscard.iso > /dev/sdc

Done.

e. boot from your USB drive and go through the same steps as for a CD.  
Plug it in, then start your computer. Hit F12 (or F10 or del or F2) during the bios start-up to select boot medium. Choose to boot from usb. Older BIOSes can't boot from USB.

Fonte: http://verahill.blogspot.pt/2012/02/installing-debian-on-usb-stick-live-usb.html