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 202MiB 0:00:06 [6.56MiB/s] [ <=>
Remove the resulting file .
$ rm delete.me