Unmount external disk after TimeMachine end backup

ubackup.sh

#!/bin/bash

VOLUME=''

while true;
do
        # Detect TimeMachine status
        STATUS=$(tmutil currentphase)
        if [[ "$STATUS" == "BackupNotRunning" ]]
        then
                # TimeMachine stop
                if [[ "$VOLUME" != "" ]]
                then
                        # TimeMachine and backup
                        diskutil unmountDisk "$VOLUME"
                        VOLUME=''
                fi
        else
                # TimeMachine working
                # Save backup disk path
                VOLUME=$(tmutil destinationinfo | egrep '^Mount Point  ' | sed 's#^Mount Point *: ##g' | head -1)
        fi
        sleep 10s
done