In magnetic media, you would hope a track in the media is perfectly
concentric with the axis of rotation. Hard drives have embedded
servo information (user data and info identifying the track, are
interleaved in the track). When you write to the media, the servo
information is fed to the servo control system, to keep the head
centered above the track.
After a so-called "secure erasure", the information in the center of
the track has been wiped, but on the "shoulders" of the track, there
could still be some information. To extract the information would
require one of two approaches:
1) If the technology allows, push the head assembly "off-track",
and read the field on the edge of the track. A modern hard
drive would make this difficult, since when you get on the edge
of the track, you might also have a reduced amplitude signal
from the embedded servo information. The drive might not be
too happy about this. Pushing a head off-track is easier with
some older disk technologies, where one platter is reserved for
servo information. Some older drive technologies actually
had commands to push them off track.
2) Remove the physical media and scan it with one of many devices
having the resolution of an electron microscope. In theory you
could develop a detailed magnetic image of the entire platter,
and by examining the field strength, find the center of tracks
and the edges of tracks etc.
Neither of these methods is that practical.
If you wish to have some notion of security, there are hard
drives with hardware support for encryption. This one for
example:
http://www.seagate.com/newsinfo/newsroom/success/D2g42.html
http://www.seagate.com/content/docs/pdf/marketing/PO-Momentus-FDE.pdf
The data at the head level is encrypted and looks like "noise"
to the casual examiner. You need the key to get the info back.
The trick then, is to make sure the key is not stored on the
machine itself, and that you make a habit of storing the key
and the computer, in different places. You can afford to lose
the drive itself, without too much worry that the info on the
drive will be recovered.
The reason that Seagate product is desirable, is there are
no "unencrypted" copies sitting around. You might also consider
using something like PGP to encrypt the files before transporting
them, but then there might still be unencrypted versions sitting
in temp directories and the like. The advantage of drive level
encryption, is everything on the drive (temp directories and
all) is encrypted at all times, until pulled into computer
memory.
Paul