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I am about to sell my old desktop PC and I am cautious about some of my sensitive information being available to the purchaser, even after reformatting the hard-drive, using data recovery software.

How can I securely wipe the hard drive so that the data on it cannot be recovered?

Although I specifically want help with my Windows PC, it wouldn't hurt if there were suggestions for Macs as well.

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Note that a reformat is indeed absolutely trivial to recover data from. You should definitely run some kind of wipe utility (that I'm sure other answerers will suggest :) – Blorgbeard Jul 16 '09 at 6:07
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Hence why I asked the question – David Pearce Jul 16 '09 at 6:08
Sorry, I thought your wording might have implied you weren't sure. I mostly wanted to make it nice and clear for anyone else reading :) – Blorgbeard Aug 2 '09 at 11:14
Here is a recipe for the Macintosh that doesn't involve destroying, or even formatting your hard drive: [macmad.org/blog/2010/10/clean-private-data-before-selling-a-mac/… "macmad.org/blog/2010/10/clean-private-data-before-selling-a-mac/…;) – Jamie Cox Jan 20 '11 at 14:49
Here is a post on IT Security talking about that: security.stackexchange.com/questions/5749/… – Shadok Mar 21 at 9:42
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10 Answers

up vote 40 down vote accepted

Look into Darik's Boot and Nuke. It's a bootable CD which lets you securely erase your hard drives.

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Good old 'DBAN' as they call it. Great to have around. – John T Jul 16 '09 at 7:39
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It says "DBAN will automatically and completely delete the contents of any hard disk that it can detect." Maybe the wording is just confusing: can I pick a single drive for it to erase? (I want to wipe an external drive, not trash my whole computer.) – Nathan Long Apr 25 '10 at 2:55
I like the DOD options for the paranoid but prepare to just waste a day letting the computer do its work... – wag2639 Jun 22 '10 at 8:30
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@GorrillaSandwich: DBAN, as I remember it, is meant for you to erase internal hard drives and it runs from the CD as a boot option. You may need to ask your case as a separate question – wag2639 Jun 22 '10 at 8:32
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@GorrillaSandwich: DBAN lets you select which drive to wipe on startup. There is an option to just wipe everything, but AFAIR it's not on by default. – sleske Jun 22 '10 at 10:46
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up vote 17 down vote

Windows7 has a tool called cipher.exe, which can wipe the disk:

http://www.ghacks.net/2010/06/21/wipe-yopur-drives-securely-with-a-hidden-windows-7-tool/

The command is simply cipher /w:x:\folder where you would substitute
x:\folder for the location you want wiped, for instance your D:\ 
drive or your C:\Users\Mike Halsey\Music folder.
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I tried 2 other solutions and this one (so far) is the simplest and good-enough-est. – jcollum Dec 14 '10 at 7:10
As mentioned in a comment to the article, and in the documentation of cipher, the tool itself does not delete anything. You have to delete all data first, and then cipher will overwrite all available space with zeros and ones. – Björn Pollex Aug 26 '11 at 18:12
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if cipher overwrites stuff ... then deleting it first is the wrong approach. just point cipher to the folder / files, it overwrites the used space (which is known since the files are still connected to the directory) with random noise ... and after cipher is done you would just delete the (now filled with random noise) file. deleting them first is wrong imho. – akira Aug 27 '11 at 12:26
@akira: It overwrites blocks marked as free by the filesystem (but which might still contain traces of deleted files, swapfile content, etc.). Deleting the files first would be correct. – Ben Voigt Oct 17 '11 at 15:22
@BenVoigt: essentially it does not matter: if you overwrite a known file (by name) sdelete knows the blocks to overwrite. if you want to clear the "free space" then sdelete first tries to allocate a huge file, which occopies all available, remaining blocks, fills them with garbage ... and free the garbage file again. – akira Oct 17 '11 at 16:09
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up vote 12 down vote

As a side note on scenarios when you cannot always format and wipe the disk and have to relinquish ownership of a computer with OS intact (like resigning a job and returning the laptop), Scott Hanselman has noted down a checklist on activities to carry out before giving it up forever.

http://www.hanselman.com/blog/TheDevelopersQuittingYourJobTechnologyChecklist.aspx

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up vote 9 down vote

An alternative might be to buy a new hard drive and do a clean install of the OS on that. You can then keep the old hard drive for a while in case you forgot to back something up and eventually recycle it as a second (or third!) drive in your new machine.

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up vote 7 down vote

The US National Institute of Standards and Technology has some Guidelines for Media Sanitization (PDF). There is a section about purging hard disks:

Purge using Secure Erase. The Secure Erase software can be download from the University of California, San Diego (UCSD) CMRR site.

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Tried it. Problematic. – jcollum Dec 14 '10 at 6:39
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up vote 5 down vote

Another suggestion, for Macs, is to use Apple's Disk Utility program. It's included on the OS X install disc, so if you boot from that, you can open Disk Utility, select your drive, and erase it (there are some options for how many times to write over data and such).

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I think this falls under the "even after reformatting" part of the OP's question. That is, it's not secure enough. – Telemachus Jul 16 '09 at 10:35
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The "options for how many times to write over data" actually let you securely erase a drive. You can choose between "overwrite with zeros" (unsecure), "overwrite 7 times" (DoD 5220.22-M-Standard) and "overwrite 35 times" (DoD 5220.22-M-Standard * 5?). – lajuette Jun 22 '10 at 7:48
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up vote 3 down vote

If you REALLY want to be sure, run the disk through a bansaw. :-)

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Then you couldn't sell it. – D'Arvit Jun 22 '10 at 8:58
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@D'Arvit: if you package it nicely ... :) – akira Dec 14 '10 at 12:39
Yeah, not trying to fun my $150 SSD through a band saw :) – Chris Dutrow Feb 15 at 17:13
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up vote 3 down vote

Well, using tools such as DBAN or the like is not only pointless, it is also very time consuming. You do NOT need to do anything else but fill the drive with 0x00 (zero bytes / NUL) once and only once, in order to prevent the recovery of any former data. Doing multiple passes is excessive and useless, let alone filling the drive with random data. The only way you can try to somehow restore anything, after such an operation took place, is with an atomic microscope - this is, obviously, an extreme procedure, that will take months for even the smallest JPG file and the error rate (false-positives) is going to be immense (in other words - you won't get anything meaningful out of this). This is even more true for higher capacity models (higher density platters).

So, the ultimate software way (fast, reliable & very secure), is a single run (zero-fill) of dd:

dd if=/dev/zero of=/dev/sdX bs=1M

However, there is something called secure erase. This is an established ATA standard. This function is integrated into the drive itself. Not only is it even faster than a run of dd (as it's already hardware based and hardware > software, speed wise), it is also more secure, due to the ability of purging original bad sectors that have been reallocated! There are 2 versions: the vanilla (2001 and onwards) and the enhanced one (post 2004). So, if your drive was manufactured roughly 10 years ago - it already supports this feature.

hdparm --security-erase NULL /dev/sdX
hdparm --security-erase-enhanced NULL /dev/sdX

Erase away!

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Just to complete the answer: To run the dd command just boot any Linux distribution from a cd or usb-stick. I can suggest Knoppix, Knoppix-based distributions and grml. – jofel Feb 23 at 9:06
Most people don't realise this. The old guttmann paper most modern wipe software reference assumed you did not know the drive type, and much bigger domains. – Journeyman Geek Feb 23 at 10:05
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up vote 1 down vote

Also, if you happen to have lots of important data, the easiest and fastest way can be to physically destroy the medium. A sledgehammer blow is much simpler than overwriting 1TB disk 15 times with random binary patterns.

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And so much more fun :p – Svish Jul 16 '09 at 7:49
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i don't think he'll be able to sell that as 2nd-hand goods if he takes your advice. – icelava Jul 16 '09 at 7:58
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"Complete PC for sale. Except hard disk gone." does not sound like a wholesome deal ;-) – icelava Jul 16 '09 at 10:31
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"hard drives slightly used". There. – Manu Jul 29 '09 at 23:34
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@brice - if there are people who are able and willing to reconstruct your hard drive after you've physically shredded it in order to get your info, then you shouldn't be asking questions like this on a public site, or going outside where the satellites can see you, either. But yes, by all means, soak the platter in acid, then shred it, then burn the shreds, then grind the ashes into powder, then sugar-coat the powder and feed it to insects. – Nathan Long Apr 25 '10 at 2:51
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up vote 1 down vote

If you have access to (or are willing to buy) the proper equipment, you may want to consider degaussing the drive. Some organizations require this before machines can be re-used, sold, or given away.

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