Shred is also a GNU command line utility to destroy data on
a magnetic medium (hard disks or floppies).
This utility is the computer counterpart of a mechanical paper
shredder.
It works by overriding several times (25 by default but you can use
the -n argument to do it more times) your data with special data
patterns designed to do the most damage on the original data.
You need to do this because; when you erase a file on most file
systems, the data of the file isn't touched at all on the medium,
only the file's entry on the directory is erased. Also, most
magnetic mediums have a faint "ghost memory" of previously written
data.
Beware, some kind of file systems do not override
data in place (like NFS or journaled file systems), shred is
useless on those, and it won't even display an error message.
Also, you must remember to destroy other copies of your data (like
backups). And if you won't need the medium anymore, and your data is
disclosure-sensitive enough or you're a paranoiac enough person,
physically destroy the disk.
If you are planning to destroy a hard disk, do the following,
unmount the protective hull (most hard disks have strange-headed
screws, but you can use a drill), carefully dismount the step
motors, the arm and the electronics board, and send them to
me (so I can have some fun with them), and destroy the disk's plates. Just shredding them may not be enough, be
sure to throughly destroy them, do not let any single piece
of flat metal alive, melt them or acid burn them.