another gpt vs mbr (sanity) check

[Scot Hetzel]

On Mon, Mar 8, 2010 at 12:16 PM, Scot Hetzel <swhetzel at gmail.com> wrote:
> On Mon, Mar 8, 2010 at 11:48 AM, Pete French
> <petefrench at ticketswitch.com> wrote:
>>> To clarify: the protective MBR is there only to protect the GPT
>>> disk from tools that do not understand the GPT. Any GPT-aware
>>> tool will treat the disk as a GPT disk. Consequently: the MBR
>>> is inferior to the GPT...
>>
>> The queston is then, why isn't Windows treating it as GPT ? It's
>
> I have both MBR and GPT formated drives installed in my laptop, and
> Windows 7 correctly identifies both drives.  The GPT drive was
> formated using FreeBSD 8.
>
>> all very nice in theory, but if the practical upshot is that
>> we get flash media which will read perfectly well in all operating
>> systems except for BSD then thats not a good result, even if it
>> is theoretically correct. Which is the situation the original poster
>> appeared to have.
>>
>
> The problem the original poster was having is this:
>
> Format drive as GPT
> Re-Format drive as MBR
>
> FreeBSD still sees the drive as GPT instead of MBR.
>
> The reason is that FreeBSD is still detecting the Seconday GPT which
> is located at the end of the disk, and using it due to the Primary GPT
> was lost/corrupted when the drive was reformated as MBR disk.
>
> The real question is how was the drive reformated.  If it was
> reformated using gpart:
>
> - Delete all existing partitions:
> gpart delete -i X adY
> - Destroy the GPT formated drive
> gpart destroy adY
> - Create the MBR formated drive
> gpart create -s mbr adY
>
> And the secondary GPT header still exists, then it is a FreeBSD bug.

I just had a look at the code for sys/geom/part/{g_part.c,
g_part_gpt.c} and it does show that we delete the secondary GPT header
when destroying a GPT formated drive.

> But if it was reformated using a different tool, then it is a bug in
> that tool for not reconizing that the original disk was formated as
> GPT.
>
> Scot
>