Force disk with old GPT label to be recognized as MBR one

Warren Block wblock at
Thu Aug 30 13:52:52 UTC 2012

On Thu, 30 Aug 2012, ??????? ??????? wrote:

> Hello there, people!
> I have a FreeBSD box with 9.1-RC3 i386 and an interesting HDD.
> It was patitioned with GPT and zfs some (long) time ago. After that
> the hdd has been completely repartitioned with MBR scheme and one
> single freebsd (165) partition and one slice there. It worked fine in
> 7.2, but now i have to get the data from that slice and when i connect
> the hdd to 9.1 box, it finds old corrupt GPT label (i suppose, it's
> backup GPT header somewhere in the end of actual disk) and does not
> recognize the MBR scheme there.

In case it has not been said: make a full backup of everything on that 
disk first using dd.

Erasing the last 34 blocks or 17048 bytes of the disk will erase the 
backup GPT.  It may be enough to erase just the very last block.

   # diskinfo -v /dev/ada1
 	512         	# sectorsize
 	256060514304	# mediasize in bytes (238G)
 	500118192   	# mediasize in sectors

   # : Not tested, could be off by one!
   # dd if=/dev/zero of=/dev/ada1 bs=512 seek=500118191

Force a retaste with 'true > /dev/ada1', then see what gpart shows.  If 
the drive is still seen as GPT, adjust the seek to begin at (mediasize 
in sectors) - 34.

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