In case you ever need to delete securely HDDs/USB of your computer, then run the folowing
shred -fuvz /dev/sda
sda – in case you have SATA drives
f – force
u – remove files after shredding. By default ‘shred’ will only shred files but will not remove them
v – verbose – show progress
z – write ‘zeros’ at end to hide shredding
shred -fuvz -n 25 /dev/sda
will last very long, and it depends on size of HDDs.
For orientation, when I shredded my 80 GB HDD it took around 10 hours with n=25 iterations ( overwrites )
You can specify n to lower value, and adapt it to your needs, default value for number of iterations is 3.
For more options see