gregjohn wrote:
> "Gail Shaw" <initialsurname@sentech sa dot com> wrote:
> 
>> Um, actually...
>> http://www.pcworld.com/article/id,108831-page,1/article.html
>> http://www.guard-privacy-and-online-security.com/how-clean-off-the-hard-driv
>> e.html
>>
>> Your boss is right. It may not be as easy as 'undelete' but the data's there
>> and it doesn't take a microscope to find it, and it could be worth a lot of
>> money.
> 
> 
> This has always bothered me.  Something does not hang right.
> 
> Could one just copy a couple of version of a linux distro (0.6 to 4 GB) on top
> of it?  Would THAT be enough to completely obfuscate what was originally on the
> disk?   If not, then why hasn't anyone yet made a commercial use of the ability
> of HDD's to remember everything that has ever been on them?
> 
> 
> 
> 

Because to go back through the information that used to be on the drive 
takes advantage of the flaws in the drive it self. The read/write head 
goes over the drive and writes a pattern, let's say 1111 overtop of what 
used to be 0101. While the drive it sending out digital data, the 
magnetic pattern on the disk doesn't exist in just a 0 or 1 state.

Think of it as just a bar refrigerator magnet, facing up/down | is one, 
sideways - is 0. but the bar could be oriented like / or \. Given that 
those don't appear to be past 45 degrees off vertical, I'd guess they 
would both read like 1 instead of 0. Now, you have a visual of a 
collection of bar magnets that aren't lined up perfectly. To write them 
all to 1s, you would just rotate each one to a more vertical position. 
Knowing the strength of the write head's magnet, you could guess how 
much that change would be. So, you read the drive head in a less digital 
and more analogue manner, and then remove the affect of the last write 
phase. The more accurate the initial reading and the knowledge of the 
write magnet's properties, the better you can guess past that first phase.

The drive it self just doesn't work that way. After a point, it's a 
statistical guess as to how accurate the data you get back is. But some 
people are willing to spend a lot of money on just a guess for certain data.

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