On Friday 06 August 2004 15:36, Tomas Tudja wrote: > I was thinking and studing about the disk wiping topic and find out > some things... > > Disk pattern is a ferromagnetic plate. "Empty spaces" are positions > on this plate, where a disk heads are writing the bits. The bits are > writing by very simple process - there is a little electro-magnet on > the disk head, which is activated by electric drive (I= ~10 to 1000 > uA) and on a position, where the bit have to be writen appears a > "permanent" magnetic mark, which means "1" (if there is no mark on > the position, that means "0"). > > We now a electromagnetic problematics called "hysteresy loop", which > is telling us: If we try to unmark the marked position on a disk > pattern (which was magnetticaly marked before), there will be a small > magnetic mark left on the "empty place", which will be polarised by > other way (90 degrees forward) and will have a 6 to 10 times smaller > level as the old mark. This mark is small, but, of course, can be > find on a pattern. There is no way to unmagnetise the ferromagnetic > material once magnetised. If somebody (probably no one :>>>>>) want > to know more about hysteresy loop, please e-mail me privatelly, > because it is Off Topic. You probably learned about hysteresy loop, > if you did study on electrical high school (in Slovakia - 2nd and 3rd > year on Railway School of Slovak Republic). > > That all means: If you write a magnetic mark on a disk pattern once, > it stays there for allways, just will be smaller and polarised by > other way (90, 180 or 270 degrees). > > So you probably can know allways, which positions was ever used for a > bit providing an information about "logical one". That probably means > that it is the way to find out what was the vallue of a bit on a disk > before it was overwriten with another vallue, but it is necessary to > use very, very, very sensitive equipment. > > I am trying to find out something about, how is the magnetic mark on > a disk pattern changing in according to time and if it is possible to > determine, WHAT mark was on a position in WHAT time. IMHO it is > possible to recover data from a wiped disk only if that is possible. > > Thank You > > PS: Sorry, I was just wondering, how simple it is, so I wanted all of > you to know it too :> > > Tomas Tudja > 136 Church Road > N17 8AJ > London, UK > [EMAIL PROTECTED] > http://hysteria.sk/catcher Basically correct and if you write the disk only once and erase it once then there is no problem in recovering the data in fact is can be done with only a high gain amp. The problem is decideing when the mark was made if it has been recorded over several times. While 30 to 40 rewrights is overkill one must consider what is being used for the source. If you use 01010101 and then 10101010 a much lower number would suffice. but with real data 00000000 is pretty common and even 40 times is not sufficient if the data is all 0's. Therefore a word or phrase may exist for a lot of overwriteing unless its done correctly. You would need a very precise writeing tool to accomplish that. -- Regards; Hoyt Registered Linux user #363264 http://counter.li.org
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