There are many Urban Legends (maybe this ought to be in the Crypt 
FAQ?) about what is actually sufficient to clear or declassify 
magnetic media when used for classified data.  Here is some 
information "from the horse's mouth".

(Regarding the sufficient overwriting of media ("clearing") meant to 
be *retained* within the controlled environment, or declassifying 
the material to be reused *outside* the controlled environment,)

From National Telecommunications and Information Systems 
Security (NTISS) "Advisory Memorandum on Office Automation 
Security Guidelines"  (NTISSAM COMPUSEC/1/87):

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"7.6.2.1 Clearing of Magnetic Media

Certain types of removable media (e.g., magnetic tape, floppy disk, 
cassettes, and magnetic cards) may be cleared by overwriting the 
entire media one time with any one character.  Floppy disks may be 
cleared by applying a vendor's formatting program that overwrites 
each location with a given character.

Fixed media (e.g., Winchester disks) should be cleared by overwriting 
at least one time with any one character.  One way to do this is by 
applying a vendor-supplied formatting program that overwrites each 
location on the disk with a given character, if it can be shown that 
this program actually works as advertised.  The user should beware: 
some programs that purport to overwrite all locations do not 
actually do this.

Cleared media may be resides within the controlled facility or 
released for destruction; however, they should be marked and 
controlled at the level of the most restrictive sensitivity of 
information ever recorded.

7.6.2.2 Declassification of Magnetic Media

Certain types of removable media can be declassified using a 
degaussing device that has been approved for declassifying media of 
that type.  (A list of approved devices is maintained by the NSA.)

If a fixed medium (for example, a hard, or Winchester, disk) is 
operative, an approved method of declassifying the disk pack is to 
employ an overwrite procedure which must overwrite all 
addressable locations at least three times by writing any character, 
then its complement (e.g., binary ones and binary zeros) 
alternatively.

When fixed  media become inoperative, it is impossible to declassify 
the media by the overwrite method.  In this case, there are two 
alternate procedures that may be used: (1) disassemble the disk 
pack, and degauss each platter with the appropriate approved 
degaussing equipment; and (2) courier the inoperative media to the 
vendor's facility, have the magnetic media (e.g., disk platter) 
removed in sight of the courier and returned to the courier for 
destruction at the secure site.  The vendor can then install new 
platters and repair any other problems with the disk unit.

7.6.3 Destruction of Magnetic Media"

[see DoD Computer Security Center, "Department of Defense Magnetic 
Remanence Security Guideline", CSC-STD-003-85  FOR OFFICIAL USE 
ONLY]

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This and many other interesting documents (many that are more 
informative than probably intended) on computer security procedures 
can be obtained (free) from:

Executive Secretary
National Telecommunications and Informations
  Systems Security Committee
National Security Agency
Fort George G. Meade, MD  20755-6000

Write them!  It's fun to be on the NSA's mailing list... 
