On Jan 27, 2009, at 9:25 PM, Bill Connelly wrote: > Why isn't "1 Pass", Writing Zeroes, sufficient to wipe a disk clean?
There exists special hardware/software that can scan a disk for the strength of the magnetism of each individual bit. There is a residual magnetism each time a bit is erased. As an example, if the residual was 10% (a convenient size for this example), and the initial bits were 0 and 1 (relative values), then the first write would be 1, and the first erase would be 0.1 (10% residual charge). The second write would be 1.1 and the second erase would be 0.11. After a series of reads & writes, the actual value of the magnetism of each bit retains a "record" of its past values within the "exact" magnetism. In this artificially convenient example, the values will only be 1's or 0's, so if you measured the value of a bit as 1.0110111 this would tell you that the current value is 1, the the previous seven values were 0,1,1,0,1,1,1 respectively. It's not this simple in real life, but "theoretically" you can reconstruct erased and rewritten data. > Isn't that "7 Passes", a DoD (Department of Defense-iveness) > government hoopla/hangup/thingy? I've read somewhere the CIA or someone had the precision to get the previous 13 read/write cycles. I'd guess this is "really difficult", but what do I know, perhaps it's been automated? --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed Low End Mac's G3-5 List, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To unsubscribe from this group, send email to [email protected] For more options, visit this group at http://groups.google.com/group/g3-5-list?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
