"Whether you are upgrading components for a system refresh,
selling an old laptop on Craigslist, or recycling your desktop PC,
there comes a time when you have to wipe a storage device in
preparation for giving it to someone else. Chances are, you don't
want whoever gets physical access to your old hard disk to have
unfettered access to its contents: files, email, passwords, the
embarrassingly-high frequency of Justin Beiber sites in your
browsing history. You'll find several open source utilities for
wiping away personal data — so this weekend, why not take
stock of your options and put together a deep-clean routine?
"Gone but Not Forgotten: When Delete is Not Delete
"One thing most experienced Linux veterans know is that once you
delete a file with rm, it's not just moved to hidden "recycle
basket" folder, it's gone. That's because Linux and other Unix-like
systems reuse filesystem blocks with regularity, and rm actually
frees the blocks where the deleted file used to reside — so
the odds are that they will get overwritten with temporary data
"But there is no guarantee that the blocks will be overwritten.
As a result, just rm-ing the contents of your disk alone won't
remove the files contents, especially if you unmount and unplug the
now-empty disk, rather than reuse it elsewhere. In fact, that's how
file-recovery tools like PhotoRec work: they scan the actual
contents of the blocks, which can include data and pointers to
other blocks of data."