echo "Unmount sdcard, save filesystem location"
set -- `sudo mount | grep sdb1`
# Answer: "dev/sdb1 on /media/joerg/rootfs type ext4 (rw,nosuid,nodev,uhelper=udisks2)"
DEVICE=$1
MOUNTDIR=$3

sudo umount $MOUNTDIR
echo "Overwriting free space with zeros ..."
sudo zerofree -v $DEVICE

sudo fsck $DEVICE

echo "Now re-plugin the SDcard to mount it again!"