Deleting unused harddisk blocks

sausagejohnson · 06-28-2005, 06:34 PM

I came across an interesting tidbit regarding erasing the sections of your harddisk that are unused. The command is:

dd if=/dev/zero of=/tmp/deleteme bs=8M
rm /tmp/deleteme

Does this work because it is getting 0's and writing a large file of 0's by choosing sections of the harddisk that are marked as unused, effectively filling up the entire harddisk before you delete the file?

Am I reading this right?

Noth · 06-28-2005, 11:41 PM

Pretty much. But to be useful you would have to do it at least a half-dozen times, overwriting data on a hard disk once isn't enough to make it non-recoverable.

sausagejohnson · 06-29-2005, 12:27 AM

Why isn't once enough? I can't see any technical reason why.

After running over each partition, I managed to get the 80GB drive down to just 118MB. Much better than I could have hoped for.

Noth · 06-29-2005, 12:32 AM

Quote:

Originally posted by sausagejohnson
Why isn't once enough? I can't see any technical reason why.

Because of the physical properties of the platters, even after you've overwritten data once or twice the original data can still be read in a clean room with the proper equipment.

Matir · 06-29-2005, 01:04 AM

I don't know that 'zeroing' blocks can reduce blocks in use, as the original poster seemed to suggest.

Noth: you are correct, it COULD still be read, but only by someone who has a LOT of money to spend on reading it. For your average user, even a single overwrite is plenty. A single overwrite will make it impossible for any software tool to recover data from the overwritten blocks.