Plug the usb key in order to find the device location:
df -k ... /dev/sdb 244476 217968 26508 90% /media/xxx
It shows that /dev/sdb is my usb key. Then unmount it
sudo umount /dev/sdb
Download these two files:
cd /tmp/ wget http://people.debian.org/~joeyh/d-i/images/daily/hd-media/boot.img.gz wget http://cdimage.debian.org/cdimage/daily-builds/daily/arch-latest/i386/iso-cd/debian-testing-i386-netinst.iso
Copy the first on the stick (it has to be umounted):
zcat boot.img.gz > /dev/sdb
Then mount the stick and copy debian-testing-i386-netinst.iso on it:
sudo mount /dev/sdb /mnt/ sudo cp /tmp/debian-testing-i386-netinst.iso /mnt/ sudo umount /mnt/
The usb key is now ready. Plug it on the computer to install, select in the bios usb for first boot device and start the debian squeeze installation.