full-disclosure - Erasing a hard disk easily

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From: fulldisc at ultratux.org (Maarten)
Subject: Erasing a hard disk easily

On Monday 12 July 2004 21:27, amilabs wrote:

> I have a bunch of old computers I would like to donate. Old laptops with
> win 95/98 on them and some old tower desktops all circa 1995-2k.
> I would like to know if I can just use a magnet to completely wipe out the
> data for obvious security reasons. I have heard about the disk erasers like
> active kill and Perl utilities for this but with the ability of data
> ghosting etc. is this enough? Can I use a big magnet like a speaker magnet
> or something like a degasser to just wipe out the data without having to
> boot these old boxes and go through all the extra work.

Um, yes and no.  Actually: No (for what you want to accomplish).   

No you cannot use a big strong magnet.  In fact, a _very_ strong magnet is 
already inside each drive, as part of the actuator coil assembly that moves 
the head. I'm not kidding, those are real strong magnets. Barely can't get 
them off my refrigerator. (no, really...)  :-)  
To get a magnet close enough to the surface to be effective you'd need to open 
up the drive and that spells certain death for the device.  And even then, to 
erase you need an alternating magnetic field, a static one (from a magnet) 
will not erase anything. Or, at least not with feasible field-strengths...

In the meantime, there are industry bulk erasers available that alledgedly 
also can handle drives (they're quite pricey!).  But their use is solely for 
destroying data AND drive together; as the drive is totally wiped, the servo 
tracks that the factory installed are also gone.  Without them, the drive is 
utterly useless, and servo tracks can only be (re)written at the factory.

So, if you want to erase the data but keep the drives operational too, you 
have but one means left: by plain old formatting.  Depending on the level of 
security you want, you can opt for:

* DOS/Windows format (beware: this does not erase ANY data at all !!)
* Low-level format / wipe (with special tools, level depends on the tool used) 
* Linux dd overwriting using /dev/zero (fairly good if used multiple times)
* Linux dd overwriting using random data (excellent if used multiple times)
* Department-of-defense level (dd as above but lots more times (like 10+))

An overwrite with all zeros will -allegedly- not withstand a serious 
data-recovery attempt by professionals, not even when repeated.
An overwrite with alternating zeros and ones will defer almost anbody, and so 
will a random-data overwrite, provided both are repeated at least 3-5 times.
A single wipe with ones is not effective against determined people.
But all of the above will still defer an amateur (without funds).   And a DOS 
format will not defer anybody except newbies, nitwits and idiots. :-)

[ the command for dd overwrite with zeros is: dd if=/dev/zero of=/dev/hd? ]

Good luck with it.

Maarten

-- 
Yes of course I'm sure it's the red cable. I guarante[^%!/+)F#0c|'NO CARRIER