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Permanent deletion of data

Posted on 2006-06-21
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I have to delete some data from my harddisk permanently. Someone told me that data on harddisks can never really get deleted even one low-formats the disk. According to them, the most secure way to erase data is to crash the harddisk physically. Could anyone tell me why old data can still be retrieved after the whole disk are completely overwritten with 0? Does magnetic storage has a sort of "memory effect"? If it is true, ist there any other way of safely erasing data besides crashing harddisks?

And how about USB memory sticks? Are they also magnetic storage like harddisks? I guess they also have something like file allocation table on them. If I delete a file on memory sticks, does it permanently disapear? If I (low)format a memory stick, can data also be recovered as on a harddis?

Thanks.


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Question by:dataminer
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by:VortexAdmin
ID: 16953543
When deleting the first bit of the file is changed so the OS doesn't see it.  The files are still there and can be recovered by using low level software to go in and change the first bit.   The best way to permanentely delete data is for a program to go in and delete everything (the first bit) then overwrite the available space with other data.  There are software apps out there you should be able to find to do this.  Simply crashing the disc isn't technically a guarantee since crashed discs can often be recovered, but in the real world....

Of course, physically breaking the discs does a pretty good job too...
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knoxzoo earned 170 total points
ID: 16955674
To permanently erase a file, or an entire drive, download Eraser and install it.  Choose the max overwrite and it'll never be recovered.  It basically overwrites the file(s) many times with variable data, rendering it unrecoverable.  
http://www.heidi.ie/eraser/

And, yes, until it's overwritten a memory stick can be recovered.  However, it only takes one complete overwrite to erase a memory stick, since it's solid state and not magnetic media.

Of course, physically destroying a disk is a complete guarantee, but you'd have to literally destroy it, as in melt it or grind it up.  Something like Eraser or Norton's Wipe does the equivalent of the same thing while leaving the disk usable.
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by:phototropic
ID: 16955801
Take a look at this:

http://www.killdisk.com/downloadfree.htm

This is a quick utility which will permanently erase data.
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by:ss6_2000
ID: 16956903
Have a look at this software it is good
Check the web site
http://www.stompsoft.com/drivewasher.html
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by:shpark82
ID: 16958952
formatting through windows applications, windows cd, etc will not completely remove data and they will be recoverable.

using third party software which gives the option of doing a full deletion will permanently delete the data by writing 1 or 0 throughout the whole location of the data.


windows deletes the pointer to the data (so the memory pointed to becomes useable by other programs and over time, new data will be written over it)

these third party software actually go and delete the pointer but also put 1s or 0s in the data it was pointing to.

it will take much much much longer to delete the files using the latter method.
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by:dataminer
ID: 16960233
thanks for the answers.

I'm aware of the fact that a normal deletion only modifies an entry in file direcoty, causing that the real file would be easily be recovered. My problem is that I've heard even using a 3rd-party program to rewrite the taget files (once) with random data cannot really completely erase them. If I rememer correctly, this has sth to do with a certain magnetic property of harddisks. knoxzoo mentioned that "eraser" should overwrite taget files "many times" so that they are completetly in-recoverble. Why is that? Will it not be enough if we just rewrite them once?

knoxzoo also mentioned that memory sticks are not magnetic media. What are they made of then? Why does one only need to rewrite them once to completely erase data? do they also have a sort of file directory on them?
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by:VortexAdmin
ID: 16960588
Memory sticks are solid state. They retain memory like your BIOS or EPROM.  If your hard drive files are deleted and written over with other data which as I mentioned many 3rd party apps do (and others have given you examples) then you should be OK.  I don't think there's any technical reason to need to do it many times, unless you're that paranoid.  If someone knows a technical method that explains how it's possible to recover data after that's been done, I'd love to hear it so speak up. You say "my problem is I've heard even 3rd party apps..."  Who did you hear this from?  Is it something you read in a security magazine or something a buddy told you?   Grab one of the 3rd party apps that does overwrites like I mentioned at the beginning and your data will be gone.

Good luck!
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by:knoxzoo
knoxzoo earned 170 total points
ID: 16960838
No, rewriting once won't do it.  It takes as many as seven overwrites to completely eliminate the ghosts of data past.  

Here's an article for you.
http://www.computerworld.com/hardwaretopics/hardware/story/0,10801,81378,00.html?SKC=hardware-81378
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by:VortexAdmin
VortexAdmin earned 30 total points
ID: 16961315
I guess I stand corrected. However this paragraph is probably something worth reading for anyone interested in this post:

[Reprinted from ComputerWorld/PCWorld article]  http://www.computerworld.com/hardwaretopics/hardware/story/0,10801,81378,00.html?SKC=hardware-81378

"Some machines can recover data that's been written over only one or two times, however. That's where secure delete standards, such as the U.S. Department of Defense 5220.22-M, come in. According to this specification, overwriting the drive sectors three times with specific, different characters constitutes one pass. Many experts recommend seven such passes to render the data completely unrecoverable. BUT READING DATA THAT HAS BEEN OVERWRITTEN BY EVEN THE SIMPLEST SHREDDERS REQUIRES EXPENSIVE HARDWARE, SO UNLESS YOU'RE WORRIED ABOUT PROFESSIONAL SLEUTHS, SUCH THOROUGH OVERWRITING PROBABLY ISN'T NECESSARY.."

So to summarize, unless you feel the data on your personal hard drive is so important and valuable to others that you need U.S. Department of Defense 5220.22-M security, any of the 3rd party scrubbers that have been suggested will probably do.

Good luck, and watch your back~!
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by:VortexAdmin
ID: 16961379
BTW, if you're considering using KillDisk, according to the writer's tests, to make the 7 overwrite passes the "security experts" recommended on a 130GB drive would take longer than A MONTH!  (35 days)  I've got a 250GB and a 300GB in my PC, how about you?  :-)
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by:knoxzoo
ID: 16962270
What I've recommended to customers with data security concerns is that they erase the file using Eraser, set to do the government overwrite (one, or a few files at a time, doesn't take much time), then set Eraser to do a standard (single pass) free space overwrite at night, as often as they feel necessary, insuring temp files and other potential security pits are cleared as well.
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