問題は、ちょっと物理的な話になりますが、ゼロをディスクに書き込む＝磁気的にある１点をゼロの状態（ＯＦＦ）にするという作業でして、一生懸命ＯＦＦにしたにも関わらず、磁気としては、微かに前の状態を残しているそうで、元が1のところに0を書いても、実際は0.2(例えば）ぐらいの状態になっているようです。
そこで、その微かな残留磁気を読み取って（方法は知りませんが、ふたを開けて、特別な装置で読むのか、基板を変えてアナログ的に読み出すのか...）、情報を取り出すことが可能（かもしれない）というわけです。 そこで、時間はかかりますが、何回もいろいろなパターンを書き込むことによって、もともと書き込まれていたデータをわからなくしてしまおうというわけです。

以下、収拾した消去方式の基準です。

● 米国防総省 DoD5220.22-M ある値→その補数→ランダム→検証

● 米陸軍(US Army) AR380-19 ランダム→ある値→その補数

● 米海軍(US navy) NAVSO P-5239-26

望ましい方式 FF(16進数)→001001-(27連続)-1(2進数)→ランダム→検証

通常方式 ある値→その補数→ランダム→検証

● 米空軍(US AirForce) 00(16進数)→FF(16進数)→ある値→検証（10%）

● グートマン推奨方式 35回の書き込み

DoD5220.22-M?
Supplement 1?
Feb-95

Overwrite all locations with a character, its complement, then with a random
character.?
Verify that all sectors have been overwritten and that no new bad sectors have
occurred.?

---------------------------------------------
●米陸軍(US Army)
http://www.fas.org/irp/doddir/army/

AR380-19
Information Systems Security?
27-Feb-98

Overwrite all locations three times (first with random charactor, second time
with a?specified charactor,third time with the compliment of the specified
charactor).

---------------------------------------------
●米海軍(US navy)
http://www.fas.org/irp/doddir/navy/

NAVSO P-5239-26?
Remanence Security Guidebook
Sep-93

(1) The preferred method for disk data storage media that may be purged by
overwriting is?to:
(a) Write all 1's to every block.
(b) MFM: Write a "1" in low order bit; a "0" in the next most significant bit;
and "1"s in?the remaining bits comprising the data block. RLL: Write
"0010011111..1111" (least?significant bit ... most significant bit) for 32 bits
and repeat this pattern throughout the?data block. Repeat the appropriate
pattern for all addressable data blocks.
(c) Write a nonlinear pseudorandom bit sequence to all locations. This sequence
is not?predictable without knowledge of the generating algorithm. One such
sequence is the output?of an encrypting algorithm (e.g., Data Encryption
Standard). In this application a biased?algorithm (i.e., producing more zeros
than ones (or vice versa)) is acceptable. The sequence?should not repeat at the
same offset on any two blocks on the disk drive. A different?starting point
for the pseudorandom bit sequence should be used for each disk drive.
(d) Verify the overwrite by reading the last data written to the data storage
media. You?should read nothing but the pseudorandom sequence.

(2) An alternate method when the required data patterns cannot be generated is
to use the?following simpler but less effective technique:
(a) Write a single character (e.g: hex EB) to all addressable locations.
(b) Write the previous character's compliment (i.e., hex 14) to all addressable
locations.
(c) Write a random character (e.g., hex 5C) to all addressable locations.
(d) Verify the overwrite by reading the last data written to the data storage
media. You?should read nothing but the random character.

---------------------------------------------
●米空軍(US AirForce)
http://cryptome.org/afssi5020.htm

AIR FORCE SYSTEM SECURITY INSTRUCTION 5020
REMANENCE SECURITY?

20 August 1996?

3.3.1. Clearing. Functioning sealed drives and Bernoulli cartridges may be
cleared by overwriting all addressable locations with binary zeros
(i.e., 0000 0000) then binary ones (i.e., 1111 1111). Then, overwrite all
addressable locations with any character (i.e., "a"). Verify the overwrite
procedure by randomly re-reading (recommend 10%) the overwritten information
to confirm that only the overwrite character can be recovered. This media may
also be cleared using a Type I1 degausser.?

---------------------------------------------
●ゲートマン推奨方式
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html
Secure Deletion of Data from Magnetic and Solid-State Memory

35 times.