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Is using multiple passes for wiping a disk really necessary?

I read some answers on a question that said that :

After deleting some data from a hard disk - Doing three passes is considered Department of Education 'secure'. Anything over 10 passes is Department of Defense 'secure'.

Would someone please elaborate.

Source : How do computers permanently store data?

I found no options to ask my question on previous question itself.

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Oh, thats cause SU isn't a forum and dosen't work like one. Assuming it hasn't been asked before, (which i could swear it has), and with a bit of cleanup, It should be a question thats alright. –  Journeyman Geek Nov 22 '12 at 7:51
scientific paper about the issue: vidarholen.net/~vidar/overwriting_hard_drive_data.pdf –  akira Nov 22 '12 at 9:11
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marked as duplicate by Karan, soandos, Hennes, TFM, techie007 Nov 22 '12 at 17:40

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

2 Answers

up vote 1 down vote accepted

In the previous answer I mentioned that to wipe out a file, you use a special tool that goes and specifically fills the place where the file was stored with 0s or junk. The number of passes is the number of times you repeat this over the same area. For instance, at this post you can see that Eraser allows you to select how many passes you want.

Frankly I myself am not too clear on the details, let me find some references.


It looks like the need for multiple passes MIGHT be a myth, atleast as far as modern hard disks are concerned. You can see this site "for a modern drive, one wipe will do just fine"

Wikipedia also agrees, according to it >1 pass is necessary for a floppy disk(remember those?), not a modern hdd.

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Thanks got my answer. :) –  Amber Nov 22 '12 at 8:45
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The idea behind multiple passes is that it is (was) conceivable that you could (with very sophisticated equipment) recover data from "residual magnetism".

Consider this - lets say you have 10101100 at a point on a drive, and you overwrite it with a random byte - say 01101110. The idea is that where two 1's align, the result will be slightly more magnetised than where a 1 and a 0 align, and where a 0 and an 0 align, there would be slightly less magentism.

So with very sensitive equipment, you could work out what the original data looked like after a single pass.

The success rate even under ideal conditions would be tiny in the past, where data densities on hard disks were such that you could examine it with this level of tolerance. But the densities now mean that it is effectively impossible.

A single pass is good enough for anyone.

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Thanks Paul, i now understand that data permanently deleted and overridden may be recovered. –  Amber Nov 22 '12 at 8:35
@Amber Actually, "overwritten" data is extremely unlikely to be recovered. –  Paul Nov 22 '12 at 8:37
This sounds like something from an 80's movie ;D –  Ярослав Рахматуллин Nov 22 '12 at 11:14
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