This article is from the CD-Recordable FAQ, by Andy McFadden (email@example.com) with numerous contributions by others.
This question comes up every once in a while, because somebody with sensitive
data wants to obliterate unwanted copies on CD-R. With magnetic media,
the problem is well understood, and guidelines have been published for
the proper treatment of floppy disks and hard drives. To the best of my
knowledge, no such guidelines have been published for CD recordable media.
To be effective and useful, an approach must have two properties: it must
guarantee that there is no hope of recovering any data from the media, and
it must be safe and easy to implement. The qualifications for the former
involve a fair degree of paranoia. If, for example, you want to erase a
file from a hard drive while leaving the remaining contents intact, it is
necessary to write over every sector in which the file was written several
times with different bit patterns. If you just zeroed out the blocks,
a sufficiently sensitive device could detect lingering magnetic traces,
and possibly reconstruct significant pieces of the original file.
Some possible approaches for CD-R:
Death by physical delamination
Scrape off the reflective layer with something sharp. Can be done by
an unskilled worker or simple device. You still need to do something
with the reflective layer, though, and there might still be traces of
data on the polycarbonate (dye residue).
Death by shredding
Run the disc through an industrial-strength paper shredder. The
polycarbonate tends to shatter into many small pieces. The resulting
jigsaw puzzle should be exceptionally difficult to reassemble.
The trouble is that the reflective layer and underlying dye is very
flexible once separated from the polycarbonate, and might not shred
well. (A much simpler variant of this is to snap the disc in half.
If you do it the right direction, the polycarbonate breaks into
several pieces. You may want to tuck the disc inside a magazine or
newspaper to control the shrapnel.) Many "home office" shredders
will handle CDs now.
Death by drum sander
Secure the disc to a piece of wood, and run it through an industrial
drum sander (http://www.performaxproducts.com/Catalog/SuperMax50x2.html).
These come with dust vacuum hoods, which should minimize the amount
of breathable polycarbonate. The system would have to be calibrated
carefully though, or the sander might just rip the data layer off and
fling it (or, for that matter, fire the whole disc across the room).
Using the piece of wood more than once might be problematic, depending
on the exact method used to attach discs to it.
Death by chemical delamination
Drop the disc into acetone. That ought to dissolve the top layer
and leave little left that's meaningful. Something still needs to be
done with the polycarbonate, though, in case it retains any traces of
the data, and disposal of acetone can be a problem.
Death by incineration
Pop the disc into a wood-burning stove. Quick, easy, effective, and
really bad for the environment. The fumes from burning polycarbonate
are not recommended as a treatment for lung disorders. Elevating a CD-R
disc above 250C (about 480F) should cause it to become fully "recorded",
but it's possible that some traces of the original recording would
Death by microwave
Microwaving a disc for a few seconds renders it pretty well unusable.
It's not clear how thorough this process is. A visual inspection
suggests that some regions of the disc go relatively untouched.
Death by coherent light
The disc was written by a laser that turned on and off. Presumably
it is possible to modify a CD recorder such that it turns the laser
on and leaves it on. This would obliterate all of the data on the
disc. It's not clear if a sensitive detector could see regions that
were "written" twice.
Death by sandblasting
Blasting discs with sand will certainly take the reflective layer
off, and do a pretty fair job of scrubbing them clean. The only
concern is for whether the delaminated layer gets fully pulverized
or just sheared off (and stays intact).
Death by sidewalk
This approach is similar to the others, but can be performed with
inexpensive equipment: a patch of rough cement and a rubber-soled
shoe. Put the disc, shiny side up, on the sidewalk. Step on it,
and twist vigorously while applying pressure. This will gouge the foil
and polycarbonate, and with sufficient force may even split the disc
itself. More force may be required on disks with adhesive labels,
and cleanup can be tricky on a windy day.
There doesn't seem to be a simple answer or perfect method. If you aren't
concerned about the NSA or a major national power recovering your data,
though, scratching with car keys or snapping in half with your hands should
be all the security you need.