Rsync two external hard drives.

sync.sh

#!/bin/bash

SRC='/Volumes/apple-a'
DST='/Volumes/apple-b'

[ -d $SRC ] || {
    echo "$SRC is not mounted";
    exit 1;
}

[ -d $DST ] || {
    echo "$DST is not mounted";
    exit 1;
}

rsync -a --progress --stats --del "$SRC/" "$DST"