Prepare bootable (USB) drive on OS X. I validated this script on a Retina MBP running Yosemite.

prepare-boot-drive-from-iso

#!/usr/bin/env bash
# Purpose: To create a bootable (USB) drive on OSX
# Note: For a NixOS 14.12 ISO (~330MB) to a 512MB USB drive, the last step (rsync) took over 3 minutes.

declare -r platform="$(uname -s)"

if [ "${platform}" != "Darwin" ]; then
  >&2 echo "Error: This will not run on ${platform} (anything other than Darwin."
  exit 1
fi

if [ "$#" -ne 2 ]; then
  >&2 echo "Error: Expected two arguments, received $#."
  >&2 echo "Usage: $0 SOURCEISO DESKDRIVE"
  >&2 echo "Example: $0 ~/Downloads/nixos-14.12.iso /dev/disk2"
  exit 1
fi

set -eu
declare -r source_iso="${1}"
declare -r dest_drive="${2}"
declare -r tmp_dir="$(mktemp -d -t nixos.XXXXX)"
declare -r source_dmg="${tmp_dir}/nixos.dmg"
declare -r vol_root="/Volumes"
declare -r vol_name="NIXOS_BOOT"
declare -r vol_dir="${vol_root}/${vol_name}"

if [ ! -f "${source_iso}" ]; then
  >&2 echo "Error: Given source ISO file does exist or is not a file."
  exit 1
fi

if [ -f "${source_dmg}" ]; then
  >&2 echo "Error: There already exists a DMG file at ${source_dmg}."
  >&2 echo "       Please remove and rereun to proceed."
  exit 1
fi

if [ ! -b "${dest_drive}" ]; then
  >&2 echo "Error: No block device exists at ${dest_drive}."
  >&2 echo "       Please confirm drive path and rerun to proceed."
  exit 1
fi

# The "magic" of hdiutil and diskutil I gained from many hours using these for bitcoin storage 
# purposes. Don't ask, but I knew there had to be a way to not have to load Ubuntu first and 
# then make the USB bootable for NixOS directly on OSX for users that don't have a physical 
# linux box laying around (for USB purposes).
# This did take a couple of hours of getting this right though despite my existing knowledge of
# hdiutil and diskutil ridiculousness. Thanks Apple. :)
function main {
  hdiutil convert -format UDRW -o "${source_dmg}" "${source_iso}"
  diskutil partitionDisk "${dest_drive}" GPTFormat "MS-DOS FAT32" "${vol_name}" "100%"
  hdiutil attach -readOnly "${source_dmg}" -mountpoint "${tmp_dir}/nixos"
  rsync -rv --progress "${tmp_dir}/nixos/" "${vol_dir}/"
  diskutil unmountDisk "${dest_drive}"
  diskutil eject "${dest_drive}"
  hdiutil unmount "${tmp_dir}/nixos"
  rm -rf "${tmp_dir}"
}

read -p "This will replace all your data on $2. Are you sure you want to proceed? (yes|no) " yn
case "${yn}" in
  y|yes)
    main ;;
  *)
    exit 0 ;;
esac

echo "Now reboot and hold down the Alt/Option then choose this USB drive to boot from."

See also these comments: https://gist.github.com/mbbx6spp/d9c3d9a23e72d78ee736 for working around the stage1 installer problem you get after you boot from USB.

I get hdiutil: attach failed - no mountable file systems, any suggestions on how to fix it?