In a message dated 6/29/2005 9:24:24 A.M. Central Daylight Time, [EMAIL PROTECTED] writes:
Just out of curiosity, why multiple passes. I know that they must have their reasons... Something like residual data on the edges of the track or some such, since there is a possibility of the track not being recorded in the exact location of the previous write? The precise location of recorded bits around the track moves around somewhat. That is why the controller does something called "read with offset" when re-reading a block that produced a data check (aka "head shaking"). The read-write mechanism/transducer is moved a tiny distance to the left of the center of where the track is supposed to be, the data is read, if still bad then the transducer is moved a tiny distance to the right of the center... If the data cannot be read correctly (i.e., no data check indication) and you have moved the transducer so far to either side that it is now within the tolerance for the adjacent track, then you give up on reading that block. Another issue is residual magnetism. When you magnetize a substance, a large % of the molecules are aligned in a certain way. When you erase that data, not all the molecules get realigned back to their original direction. This is why skilled recording experts with expensive, sensitive equipment can recover recorded data that was over-written or erased. The cost of such recovery is high. The cost of erasing data to the point that it cannot be recovered by an expert is also high. The trade-off is how valuable is the data to you and what would it cost you if someone else got the data. Here are the DoD's standards: _http://72.14.207.104/search?q=cache:dRHWf5Fg2LQJ:security.ouhsc.edu/docs/DoD_5220.doc+DoD+5220&hl=en_ (http://72.14.207.104/search?q=cache:dRHWf5Fg2LQJ:security.ouhsc.edu/docs/DoD_5220.doc+Do D+5220&hl=en) On the line that says non-removable rigid disk it says that in order to clear the data all you need to do is to overwrite all addressable locations with a single character, such as writing a full track of X'00'. But in order to "sanitize" the disk you must degauss the disk, destroy the disk, or use process "d", which means to Overwrite all addressable locations with a character, its complement, then a random character and verify. This would require at least 4 revolutions of the track, the first 3 of which are writing and the 4th is reading, then some CPU time to do the verify. So if it takes X hours to download the data and you want it sanitized, it will take 4X hours to sanitize. Bill Fairchild ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [EMAIL PROTECTED] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
