Because I tend to forget. This is a simple way to zero out all free space on a drive all that is needed is PV and administrative privileges . If PV is not installed on the system you can install it via your package manager .
sudo apt-get install pv
We will be directing all /dev/zero output to a file in the target drive we wish to zero-out . Once all free space is occupied in the target drive the operation will stop by itself .
sudo pv /dev/zero > /mnt/usb/delete.me
Sample output .
pi@raspberrypi:~ $ sudo pv /dev/zero > /mnt/usb/delete.me
Remove the resulting file .