9 Mar
2011
9 Mar
'11
9:51 p.m.
vintagecoder wrote:
I understand that and it makes sense
"deleting" data or trying to
overwrite
a filesystem record doesn't necessarily do what we
think. But filling
the drive to its capacity with a utility like dd using zeros or
random data
has to work because you can read the data back, so
it's really there.
No, that doesn't "have to work". Even though the drive is "filled to
capacity", there still can be spared-out blocks containing old user
data. The capacity that the drive reports is less than the physical
number of blocks on the medium, specifically because there are blocks
reserved for use as spares.
Right, for data that was "deleted", but new
data that was written has
to end
up on the chip somewhere...and this means it really
does replace what
was there before, unless there's extra capacity there...
Which there is.
in which case some number of passes of filling it with
random data
has to work eventually, by definition.
If you do that, eventually enough blocks will fail that the drive can no
longer spare them. It will start reporting errors to the host.
However, this does NOT mean that you have succesfully erased any
previously spared blocks containing remnants of user data. Once they're
spared out, they generally stay spared out forever, so the remnants of
user data will still be there.
Eric