|
I've been reading about various utilities that erase the free space on your hard drive, and they all talk about doing multiple passes of writing random strings of 1's and 0's using various algorithms... My question is this--why not simply write all 0's to the free space (or all 1's)? A single pass and you're done? I'm guessing I'm missing something really basic... |
|||
|
|
|
In very loose terms, to emphasize where the complication lies: The bits on the drive are interpreted as binary, "1" and "0" if you like, but in reality it is a continuous variable that is measured. One could figuratively say that every bit really can take any value between 0 and 1, and the drive interprets all values >0.7 as 1, and all values <0.3 as 0. Let's say a bit is at charge 0.9. You then overwrite it with a 0, which effectively lowers the charge. The final charge will be maybe 0.25, but if the bit originally had been a zero at charge 0.2, maybe it would end up as 0.15. Thus, by using equipment possible to read the charges at high precision, in theory one could recreate data that has been overwritten by all zeroes by using a normalization where charge<0.2 is a zero and charge>0.2 is a one. If one instead overwrites the data with random numbers, it is instantly much harder for this recreation. That's why it is preferred for very sensitive data. In reality algorithms are much more clever, depending on how good the resolution of the equipment used to analyze the magnetization of the disk is. There is a reason why the data recovery companies charge silly money :-) |
|||||||||
|
|
I think the basic idea is that the data is written to a narrow circular track on the rotating disc. The head moves to different tracks but this movement has a limited accuracy. So when the head moves to that track to write zeroes, it may not write them exactly over the top of your data. There is probably a lot of overlap, but, in theory, in a laboratory the disk platter could be put into a special drive with narrow heads that move more precisely and this way the edge of your old data might be read. Either that or, because the data is written by re-orienting magnetic "particles" in the disk, which is essentially an analogue statistical physical process, some faint magnetic "shadow" of the old data might remain which could still be detected by sensitive heads. I'm not sure that any of this is more than hypothetical. So the idea is to overwrite the data multiple times, like scribbling over your credit-card number on a piece of paper to stop anyone being able to read it. The more times you scribble, the harder it is to read what was underneath. |
|||
|
|
|
Todays forensic and data recovery techniques are accurate enough to still read the original data if only one pass of 0 or 1 is written over the original contents. |
|||||||||
|
