This script will sync an external drive with a NAS.

backup-rsync-nas-external.sh

#!/bin/bash

remote="/Volumes/Volume_1"
local="/Volumes/Storage"
LOCK=~/Desktop/Synching
logging=~/backup-rsync-log.txt
sleeptime=20
maxthreads=1

set -e
function cleanup {
    echo "Removing Synching folder"
    rm -rf $LOCK
}

trap cleanup EXIT

mkdir $LOCK || { echo "Backup already running" ; exit 1 ; }
Rez -append ~/backup.rsrc -o ~/Desktop/Synching/$'Icon\r'
SetFile -a C ~/Desktop/Synching

sleep 15 # give time for mounting

# Check if external drive is mounted
if mount | grep "on ${local}" > /dev/null; then
	echo "Storage drive is mounted"

    # Check if network NAS is mounted
	if mount | grep "on ${remote}" > /dev/null; then
		echo "Remote NAS is mounted"
		echo "Running rsync"

        # loop over all root external drive folders and do an rsync for each, up to a limit
		for dir in "$local"/*/; do
			if [ "$dir" = "Network Trash Folder" ]; then
				continue
			fi
			folder=`basename "$dir"`
			echo "Synching $folder"
            # create the folder if it doesn't exist, keeping permissions
			if [ ! -d "${remote}/${folder}" ]; then
				mkdir -p "$remote"/"$folder"
				#chown --reference="$dir" "${remote}/${folder}"
            	chmod --reference="$dir" "${remote}/${folder}"
			fi

			echo -e "\nStarting rsync $(date +%Y%m%d_%H%M%S) of" $dir >> $logging

            # don't go crazy with rsync, so limit it
			#while [ `ps -ef | grep -c [r]sync` -gt ${maxthreads} ]; do
            #	sleep ${sleeptime}
        	#done

            # start rsync parallel threads.
			#nohup rsync -avh --progress --delete "$dir" "$remote"/"$folder"/ </dev/null >>$logging 2>&1 &
			rsync -avh --progress --delete "$dir" "$remote"/"$folder"/ #& 

		done
        wait
		echo "Done"
	else
		echo "Remote NAS is NOT mounted"
	fi
else
	echo "Storage drive is NOT mounted"
fi




###########
~/Library/LaunchAgents/launch-backup.plist

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" \
"http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<dict>
        <key>Label</key>
        <string>storage.backup</string>
        <key>LowPriorityIO</key>
        <true/>
        <key>Program</key>
        <string>/Users/memyselfi/backup.sh</string>
        <key>ProgramArguments</key>
        <array>
                <string>backup.sh</string>
        </array>
        <key>WatchPaths</key>
        <array>
        <string>/Volumes</string>
        </array>
</dict>
</plist>