There are basically 3 ways to eliminate information from a hard drive
(sanitizing a hard drive):
For the non-techie, the basic file deletion provided by the O/S does not
actually remove the data from the hard drive. Data pointers and file
locations are the only information deleted. There are several methods and
software programs available that can "un-delete" these types of files. So
the only really secure method is to completely remove the data from the hard
1) Overwriting a hard drive is probably the solution that this particular
user is interested in, since they wish someone to re-use the hard drive.
There are several free, shareware, commercial products, and even services
that can provide this option. The basic concept is that the entire hard
drive is re-written with useless or random information over every track and
sector. This is not absolutely reliable, as I understand there is now even
methods that allow a forensic search of a hard drive allowing information
that has been overwritten several times to still be recovered.
2)De-gaussing (de-magnetizing) is also a way to magnetically destroy data on
hard drives. This takes a De-gaussing machine, which the general public does
not usually have immediate access to. There are service companies that can
perform this function for you, and are often used by commercial enterprises
when they have multiple hard drives to purge. Unfortunately, degaussing can
often render a hard drive unusable, if the demagnetizing affects the spindle
motor or eliminates the timing tracks on the hard drive.
3)Destruction is fairly obvious. The hard drive is physically destroyed
(smashing the platters or rendering them magnetically useless, which
eliminates all data, but also eliminates the hard drive from any future use!
The only absolutely safe method is physical destruction. If the user has ANY
reservations about someone finding their data on a hard drive and
considering the reasonably inexpensive replacement cost of a hard drive.
(information found at cnet)