use unetbootin to put iso on usb stick.
USB stick must be VFAT formatted
Once done, if space is an issue, remove all files except /boot
Copy the iso image to the root of the usb stick and unmount

Boot from USB in DEBUG mode

When the boot process is completed, you will see a message similar to 
“no cdrom found – unable to continue (type exit or Ctrl-D to reboot)“

Identify your usb device and mount it
fdisk -l

mount /dev/sdb1 /mnt (if your usb stick was identified as /dev/sdb of course)

mkdir /mnt2

mount -o loop -t iso9660 /mnt/proxmox_[version].iso /mnt2

chroot /mnt2 sbin/unconfigured.sh