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>That computer, armed with the company’s proprietary wiping software, uses a special algorithm to programmatically access each bit on the drive, stored as eithe

by bko 10y ago

>That computer, armed with the company’s proprietary wiping software, uses a special algorithm to programmatically access each bit on the drive, stored as either a ‘0’ or ‘1,’ and write a new digit to it. The process must be repeated at least three times and with different numbers to be up to DoD standards. Maybe a stupid question, but why don't they just write every bit to either 1 or 0 regardless of its current content? Are they trying to preserve some data and not others? Why do you need to do it multiple times if the bit being written is independent of the actual contents?

gizmo686 10y ago
>Why do you need to do it multiple times if the bit being written is independent of the actual contents? There is a concern that the write process is not perfect, so the old value of the bit might still be visible under a microscope.
- cr0sh 10y ago
  It's not because "the bit might still be visible under a microscope" - it's because of something called (IIRC) "intra-track data". Basically when a hard drive stores the data on the physical platter, it writes it as a series of concentric tracks. These tracks are spaced very close together. When data is written to the drive, the magnetic regions that are written on the track can "bleed over" to the intra-track regions, leaving a faint trace that with the proper equipment ($$$$) can be read, even if the tracks are all zeroed. Think of the magnetic regions as not being (say) "spikes", but rather "hills", where the greatest magnetic intensity is at the peak, tapering off as it reaches the edge/base of the hill. That energy might be small, but it is there. By overwriting each track multiple times with varying data patterns, these leftover tale-tell magnetic leftovers are essentially pushed into the noise of the rest of the data, and are effectively "wiped". But you have to write the pattern multiple times, over all the tracks. This effect has "recently" been used to an advantage (depending on how you look at it) for something called an "SMR Hard Drive" - or "Shingled Magnetic Recording": https://en.wikipedia.org/wiki/Shingled_magnetic_recording https://en.wikipedia.org/wiki/Shingled_magnetic_recording In this kind of hard drive, the tracks are purposefully placed very close together so that the magnetic regions purposefully overlap, allowing for a higher density on fewer platters (so, larger drives in a smaller space for a lower cost). However, they tend to be slower - and not recommended for purposes like RAID NAS or anything where you want speed and proper data retention (my understanding, at least).
adrianN 10y ago
Drives store analog information and only present it in digital form to you. So a 1 might be a 0.8 and a 0 might be a 0.2. Writing all ones might turn bits that already were "1" to 0.9, while leaving bits that used to be "0" at 0.8.
- 0x0 10y ago
  It is generally accepted that it is impossible to recover data from a single wipe on modern hard drives. There is some mention of it here https://en.wikipedia.org/wiki/Data_erasure#Number_of_overwrites_needed https://en.wikipedia.org/wiki/Data_erasure#Number_of_overwri... and also I believe I read somewhere that there's unclaimed prizes available for anyone who can prove otherwise (but I do not have a source available for that now). What can bite you is the drive firmware silently remapping sectors, causing your write request to be mapped to a different sector altogether.
yaur 10y ago
There was a (possibly mistaken) belief that some data could be recovered with an electron microscope. AFAIK it is definitively not true with modern spinning drives and its the "AFAIK" portion of that statement that keeps people doing multiple wipes.
- anfractuosity 10y ago
  I think you might mean something like a Magnetic Force Microscopy. Edit: It looks like there is something called lorentz microscopy that uses an electron beam.
gowld 10y ago
To make sure every bit is overwritten, they need to change all the 0s to 1s and all the 1s to 0s ;-) I'm also curious what's the third number they found in {0,1}
- Casseres 10y ago
  new ≠ different

by bko 10y ago

gizmo686 10y ago

>Why do you need to do it multiple times if the bit being written is independent of the actual contents? There is a concern that the write process is not perfect, so the old value of the bit might still be visible under a microscope.

cr0sh 10y ago
It's not because "the bit might still be visible under a microscope" - it's because of something called (IIRC) "intra-track data". Basically when a hard drive stores the data on the physical platter, it writes it as a series of concentric tracks. These tracks are spaced very close together. When data is written to the drive, the magnetic regions that are written on the track can "bleed over" to the intra-track regions, leaving a faint trace that with the proper equipment ($$$$) can be read, even if the tracks are all zeroed. Think of the magnetic regions as not being (say) "spikes", but rather "hills", where the greatest magnetic intensity is at the peak, tapering off as it reaches the edge/base of the hill. That energy might be small, but it is there. By overwriting each track multiple times with varying data patterns, these leftover tale-tell magnetic leftovers are essentially pushed into the noise of the rest of the data, and are effectively "wiped". But you have to write the pattern multiple times, over all the tracks. This effect has "recently" been used to an advantage (depending on how you look at it) for something called an "SMR Hard Drive" - or "Shingled Magnetic Recording": https://en.wikipedia.org/wiki/Shingled_magnetic_recording https://en.wikipedia.org/wiki/Shingled_magnetic_recording In this kind of hard drive, the tracks are purposefully placed very close together so that the magnetic regions purposefully overlap, allowing for a higher density on fewer platters (so, larger drives in a smaller space for a lower cost). However, they tend to be slower - and not recommended for purposes like RAID NAS or anything where you want speed and proper data retention (my understanding, at least).

adrianN 10y ago

Drives store analog information and only present it in digital form to you. So a 1 might be a 0.8 and a 0 might be a 0.2. Writing all ones might turn bits that already were "1" to 0.9, while leaving bits that used to be "0" at 0.8.

0x0 10y ago
It is generally accepted that it is impossible to recover data from a single wipe on modern hard drives. There is some mention of it here https://en.wikipedia.org/wiki/Data_erasure#Number_of_overwrites_needed https://en.wikipedia.org/wiki/Data_erasure#Number_of_overwri... and also I believe I read somewhere that there's unclaimed prizes available for anyone who can prove otherwise (but I do not have a source available for that now). What can bite you is the drive firmware silently remapping sectors, causing your write request to be mapped to a different sector altogether.

yaur 10y ago

There was a (possibly mistaken) belief that some data could be recovered with an electron microscope. AFAIK it is definitively not true with modern spinning drives and its the "AFAIK" portion of that statement that keeps people doing multiple wipes.

anfractuosity 10y ago
I think you might mean something like a Magnetic Force Microscopy. Edit: It looks like there is something called lorentz microscopy that uses an electron beam.

gowld 10y ago

To make sure every bit is overwritten, they need to change all the 0s to 1s and all the 1s to 0s ;-) I'm also curious what's the third number they found in {0,1}

Casseres 10y ago
new ≠ different