Beware i wrote this over a year ago for ubuntu 12.04+ (note it uses xvd* instead of sd*)

free_attach_point.rb

def get_open_point(existing_device_prefix, new_device_range_start, new_device_range_stop)
  # existing_device_prefix should be something like 'xvd' or 'md'
  # new_device_range should be similar to '0' and '9' or 'f' and 'p'
  possible_attach_points = (new_device_range_start..new_device_range_stop).map do |drive_letter|
    "/dev/#{existing_device_prefix}#{drive_letter}"
  end
  Chef::Log.debug("#{@new_resource} Possible Attach Points: #{possible_attach_points}")

  # Get a list of in use devices (fold numbered partitions together if looking at disk drives)
  existing_devices = `ls /dev/#{existing_device_prefix}*`.split().each do |existing_device|
    if existing_device_prefix.include? "xvd"
      existing_device.gsub!(/[0-9]/, "")
    end
  end
  Chef::Log.debug("#{@new_resource} Existing Devices: #{existing_devices}")
  existing_devices.uniq!

  # Return an array of all open attach points
  free_points = possible_attach_points - existing_devices
  Chef::Log.debug("#{@new_resource} Free Points: #{free_points}")

  if not free_points.any?
    # Fail over to operator control
    raise "No open mount points in the range /dev/#{existing_device_prefix}[#{new_device_range_start}-#{new_device_range_stop}]"
  end
  return free_points
end

def get_free_attach_point
  # Linux kernel 3.0+ uses /dev/xvd* instead of /dev/sd*
  # AWS recommends using mount points f - p, although others are available
  free_point = get_open_point("xvd", "f", "p").first
  Chef::Log.debug("#{@new_resource} Free Attach Point: #{free_point}")
  return free_point
end

def get_free_raid_point
  # If we have over 10 raids per machine, god help us
  free_point = get_open_point("md", "0", "9").first
  Chef::Log.debug("#{@new_resource} Free Raid Point: #{free_point}")
  return free_point
end