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):  ------  "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]  ------  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...