another gpt vs mbr (sanity) check

[Andriy Gapon]

on 08/03/2010 20:16 Scot Hetzel said the following:
> 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.

Not exactly true -  primary GPT (in sector 1) was also intact.
Typical MBR formatting reserves sectors 1-62, but doesn't actually put anything
there.

> 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.
> 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.

It was formatted either with GPT-unaware tool, which is not a deadly sin (or a
bug), or with a tool that simply ignored anything already on the disk by design.

-- 
Andriy Gapon