Unexpected XFS SB number 0x00000000

["Chris" <hsvchris@gmx.de>]

Hello!

I'm running a home file server with Debian GNU/Linux 4.0 4.0r1 etch
(2.6.18-5-amd64 Kernel) and an Areca ARC-1220 hardware RAID controller.
I used to have 4 750GB HDDs connected and set up as RAID 5 array, single
volume, single XFS partition (set up during the installation of Debian). No
problems so far.

Now I added another 750GB HDD to the array, online capacity/volume expansion
by the controller finished just fine.
My plan was to add the extra space to the above mentioned XFS partition. So
I unmounted the partition, started cfdisk, removed the partition table and
wrote a new one that included the new free space. 
After rebooting the partition wasn't mounted, so I couldn't use xfs_growth
to expand the filesystem.  

xfs_check: unexpected XFS SB magic number 0x00000000

xfs_repair -n:
Phase 1 - find and verify superblock...
bad primary superblock - bad magic number !!!
attempting to find secondary superblock.......[...].............found
candidate secondary superblock...unable to verify superblock,
continuing..........[...]................
.......Sorry, could not find valid secondary superblock
Exciting now.

I realize the magic number 0x00000000 is probably not a good thing and maybe
using fdisk to write a new table was not the way to do it?
Any suggestions on restoring the old partition table / magic number or how
to proceed? Is there an easy fix or is this a serious problem?

Kind Regards,
Chris