[clue] secure erase techniques?

[Drew Eckhardt]

The bs=387 doesn't do anything constructive since /dev/sda is a block device where all writes to the device are multiples of block size (generally 512 bytes), and any smaller writes are done as a read into the buffer cache, modifications of the contents there, and eventual flush.

In practice the physical writes to the device are going to be significantly larger than block size because they'll be sequential and the buffer flushing code tries to combine writes.

The number of writes which get combined might come down to how many scatter/gather buffers are supported by the controller+driver although I haven't touched the code in over a decade and can no longer remember details like that.

--- On Tue, 5/31/11, Christopher Cross <g1ccross at gmail.com> wrote:

From: Christopher Cross <g1ccross at gmail.com>
Subject: Re: [clue] secure erase techniques?
To: "CLUE's mailing list" <clue at cluedenver.org>
Date: Tuesday, May 31, 2011, 8:05 AM

I would like to know why you chose bs=387 and why you use the count option. I have always set bs to be close to the drive cache size as this usually runs quicker for me and if I am doing this to an entire drive or disk image I usually leave out the count option all together. Is my method wrong for any reason?

On May 31, 2011 8:51 AM, "Raymond DeRoo" <rderoo at deroo.net> wrote:
> Mike--
> 
>> In short, I'm giving one of my older PC's to a friend's friend.  One of those, I don't need it, things.  In any case, in terms of secure destruction of drives, my father's always taught me to disassemble the drives and throw the heads and the platter out separately.  Can't do it here, they need the drives, so I thought I'd ask for advice in case we have members who might know a thing or two about this sort of thing.  I figured I'd probably just boot it to a live CD and nuke the partitions, and that's probably enough, after all, I don't need like a military-grade erase, but I'll settle for making it @#$@#$@$ hard to recover.  Any suggestions are welcome.



> 
> 
> # dd if=/dev/urandom of=/dev/sda bs=387 count=<size of disk in bites> / 383
> 
> Recover from this *IS* still possible, but generally requires someone who is *VERY* knowledge about drives to do as such.



> 
> .r


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