@fx1024: Sure, it's only 13 lines so I'll just post it here:
#!/bin/sh
DATE=$(date "+%d-%m-%Y")
BUDISK=/dev/sdc1
BUDIR=/mnt/USB/USB1_c1/backups_platte
BUDEST=$BUDIR/$DATE
BUSOURCE=/mnt/HD/HD_a2
[ -d $BUDIR ] || { echo "Backup-Verzeichnis $BUDIR nicht gefunden, Abbruch!"; exit 1; }
df -h $BUDIR | grep $BUDISK || { echo "Backup-Laufwerk $BUDISK nicht gemountet, Abbruch!"; exit 1; }
rsync -avX --delete --link-dest=$BUDIR/current $BUSOURCE/ $BUDEST/
rm $BUDIR/current
ln -sfT $BUDEST $BUDIR/current
The script uses rsync to do an incremental backup to the USB attached disk, so you'll need rsync on the NAS (don't know whether that's available) or you'll have to modify the script to fit your needs.
@ivan: Are you sure about that? I use an USB to SATA docking station by Sharkoon and my external disk shows up as /dev/sdc and mounts as ext4, so its behaviour is identical to that of the internal disks. I think I even formatted it as ext4 on the NAS itself.