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Date: Tue, 4 Oct 2016 06:23:26 -0600 (MDT)
From: Warren Block <wblock@wonkity.com>
To: John Baldwin <jhb@freebsd.org>
cc: freebsd-current@freebsd.org, Ngie Cooper <yaneurabeya@gmail.com>,
 Ernie Luzar <luzar722@gmail.com>,
 "Hartmann, O." <ohartman@mail.zedat.fu-berlin.de>
Subject: Re: Destroy GPT partition scheme absolutely, how?
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On Mon, 26 Sep 2016, John Baldwin wrote:

> On Tuesday, September 27, 2016 12:36:22 AM Ngie Cooper wrote:
>>
>>> On Sep 26, 2016, at 22:48, Ernie Luzar <luzar722@gmail.com> wrote:
>>
>> ...
>>
>>> 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
>>
>> Why not just use "gpart destroy -F provider"?
>
> That doesn't always work.  In particular, if a disk was partitioned with GPT
> and then you use normal MBR on it afterwards, the 'gpart destroy -F' of the
> MBR will leave most of the GPT intact and the disk will come up with the old
> GPT partitions, not as a raw disk.

Right.  So do a gpart destroy -F of whatever is on there, ignoring 
errors, then a gpart create -s gpt.  Now there is definitely a secondary 
GPT, and a final gpart destroy -F removes it cleanly.