Thanks eljohnsmith and slycat for this great script and idea! Works really good.
I modified the perform-the-backup-part, so you don't have to recreate the directories when you restore. I also added /tmp and /run to the exclude option, because they're also populated at boot.
And removed the verbose flag

Thanks again!
$this->bbcode_second_pass_code('', '
#! /bin/bash
# script to create a tar backup file
# of the pogo plug device
# directory to place backups
BACKUPDIR=/media/mybook/backup/POGO
# file name YYYY-MM-DDHH:MM:SS.tgz
BACKUPNAME=`date +%F%T`.tgz
# remove dashes and colons from file name
BACKUPNAME=${BACKUPNAME//[-|:]/}
# root directory for the backup
SOURCE=/
# number of sets to keep
NUMSETS=5
# perform the backup
tar -cpzf $BACKUPDIR/$BACKUPNAME \
--exclude=/proc/* \
--exclude=/mnt/* \
--exclude=/sys/* \
--exclude=/dev/* \
--exclude=/tmp/* \
--exclude=/run/* \
--exclude=/lost+found \
--exclude=/media/* \
$SOURCE
# delete older files greater than $NUMSET
cd $BACKUPDIR
LINS=$((NUMSETS + 1))
if [[ $(ls $BACKUPDIR | wc -l) > $NUMSETS ]]
then
stat -c "%Y %n" * | sort -rn | tail -n +"$LINS" | \
cut -d ' ' -f 1 --complement | xargs -d '\n' rm
fi
')
PogoPlug v2 Pink, w/ transmission, flexget, nfs, owncloud..