#!/usr/bin/env bash

DISK=""       # Name of a directory mounted in /Volumes
INTERVAL=15

while true; do
  s1=$(df -m | grep "$DISK" | awk '{print $3}')
  echo "s1=$s1"
  sleep $INTERVAL
  s2=$(df -m | grep "$DISK" | awk '{print $3}')
  echo "s2=$s2"
  r=$(( (s2 - s1) / INTERVAL ))
  echo "$DISK Write Rate: $r MB/S"
done