Re: Destroy GPT partition scheme absolutely, how?

Hartmann, O. wrote:
> I ran into a very nasty and time consuming problem. Creating a NanoBSD
> image with a modified script framework creating GPT partitions, I put
> the imaes via "dd(1)" on USB flash or SD flash. Because the images are
> usually much smaller than the overall capacity of the USB or SD, the OS
> (FreeBSD CURRENT, recently built as of this morning) complains about
> the second GPT header isn't in the last LBA. Sometimes, my PCengines
> APU2 doesn't boot then, a relief is to issue the command 
> 
> gpart recover da1
> 
> (in that case, the USB flash drive or SD flash is recognized
> as /dev/da1).
> 
> But I run into a nasty situation, if the image put to the flash is
> somehow corrupted. Then I tried to write a second, repaired image over
> the first one using dd(1) again and do a recovering as mentioned above
> - but this is fatal in two ways. First, the corrupted/broken GPT seems
> to be "recovered" and put in replacement of the correct one - so I
> guess. Performing no recover leaves the image on flash corrupted
> anyway.
> 
> Well, to be honest, I didn't exactly know what is going on here. The
> phenomenon is that I had a problem creating a NANO_DATASIZE= DATA
> partition with an empty NANO_DATASIZE which somehow corrupted the
> whole image. The image then never booted, complaining,
> that /foo/bar/_.mnt was unmounted unleanly.
> 
> This happened multiple times, even if I tried to overwrite the SD or
> USB flash with /dev/zero or /dev/random data, but I do stop such a dd
> after a couple f minutes, since the SD is 32GB in size and the USB
> flash drive is 32 GB, 64 GB and 128 GB - a pain in the ass if you want
> to write via USB 2.0. But even with overwriting with a good image then
> results in a corrupt image on flash drive, complaining about the GPT
> second header not in last LBA and the issue with the uncleanly
> unmount _.mnt (from the creation process of the NanoBSD image)! 
> 
> So I guess there is something magic happening. Some informations are
> not lost and I suspect the "recovery" moving those foul data into
> active places.
> 
> Using a fresh/new SD or USB resolves the problem. But the question
> remains: how can I destroy any relevant GPT information on a Flash
> drive (or even harddisk) to avoid unwanted remains of an foul image
> installation? 
> 
> First guess was to write the last couple of bytes on such a flash drive
> by letting dd(1) counting backwards, but I couldn't figure out how to
> let dd(1) do such a procedure. The nightmare didn't end, while trying,
> the SD flash card died :-(
> 
> thank you very much for your help and thoughts.
> 
> Kind regards, thanks in advance,
> 
>  

This little script has been posted before. Maybe it will be what your 
looking for. Called gpart.nuke

#! /bin/sh
echo "What disk do you want"
echo "to wipe? For example - da1 :"
read disk
echo "OK, in 10 seconds I will destroy all data on $disk!"
echo "Press CTRL+C to abort!"
sleep 10
diskinfo ${disk} | while read disk sectorsize size sectors other
do
  # Delete MBR and partition table.
  dd if=/dev/zero of=/dev/${disk} bs=${sectorsize} count=1
  # Delete GEOM metadata.
  dd if=/dev/zero of=/dev/${disk} bs=${sectorsize} oseek=`expr $sectors 
- 2` count=2
done