non mountable xfs with 2 primary and 2 logical

[Richard Waterbeek <richardwbb@versatel.nl>]

Hi linux-xfs mailinglist,

I've recently subscribed to this list. I've seen many messages, but have
little understading of the 'XFS' file system. I know about mkfs.xfs.
I've benched my hardware. But, now my harddrive had a collision with the
mbr somehow. [or something]

But, now I've 'hexdumped', the first 512 bytes, [because after a
disaterous reboot, cfdisk is now saying; "/dev/sda empty".

I'm aware of two primary and two logical partitions. I feel that I know
the size of the first and second primary, but the xfs/ sda5 and, xfs
sda6, are no longer appearing. I had to '#' the /etc/fstab, because it
won't boot [long waiting].

Now I would like to restore the first 512 bytes which is the MBR.

I have absolutely no understanding of -where-, "the SuperBlock" [or how
that should be called], happens to be.

But, I feel that 512 bytes missing and no reason to believe a whole
Terabyte would be "ones and/ or zero's", now, I wonder, how to restore
this. I have not made backup of this harddrive, just an rsync, which is
a little outdated and a bit incomplete, because difference in size of
both drives.

I might be able to create a XFS-something [from /dev/sda, for example],
and write that with dd. 

But, before I would 'try this out', I would like to gain some
understanding, of what -should- be on the first 512 bytes of a
harddrive, besides what I have now, cfdisk is saying empty.

Hoping to receive a reply, Richard, the Netherlands