CentOS - Jan 2012 - Corrupt mbr and disk directory map

We are running Centos 5.6.  All was fine until yesterday.  I attempted 
to tar a 14KB work file to a USB floppy  (/dev/sdb) for transport to 
another server.  Unfortunately, I keyed in 'tar cvf /dev/sda filename' 
instead of 'tar cvf /dev/sdb filename'.   /dev/sda is our main 
(boot/root/apps) scsi hard drive.   I realized my mistake, but it was 
too late.  The system is still powered up and running, but I am sure 
that I have overlaid (and trashed) the mbr and the disk directory map.

When I run the 'fdisk /dev/sda' command, the console displays:

"Device contains neither a valid DOS partition table, nor Sun, SGI or 
OSF disklabel.  Building a new DOS disklabel.  Changes will remain in 
memory only, until you decide to write them.  After that, of course, the 
previous content won't be recoverable."

When I run grub and enter 'find /grub/stage1', the console displays 
"Error 15: File not found"

I suspect that when I power the system down, it will not reboot.  I can 
reinstall mbr and grub, but I don't have the original partition table 
start/end values.  Since the system is still running, it seems that the 
partition table must still be available to it from somewhere.

Is there any way to easily restore the partition table?

The system is completely backed up and can be restored if necessary, but 
I prefer a quick and simple solution, if possible.

Any ideas?
Thanks

Billy Davis wrote:
> We are running Centos 5.6.  All was fine until yesterday.  I attempted
> to tar a 14KB work file to a USB floppy  (/dev/sdb) for transport to
> another server.  Unfortunately, I keyed in 'tar cvf /dev/sda
filename'
> instead of 'tar cvf /dev/sdb filename'.   /dev/sda is our main
> (boot/root/apps) scsi hard drive.   I realized my mistake, but it was
> too late.  The system is still powered up and running, but I am sure
> that I have overlaid (and trashed) the mbr and the disk directory map.
<snip>
> I suspect that when I power the system down, it will not reboot.  I can
You have that right.
> reinstall mbr and grub, but I don't have the original partition table
> start/end values.  Since the system is still running, it seems that the
> partition table must still be available to it from somewhere.
>
> Is there any way to easily restore the partition table?
Easily? No (other than the grub-install /dev/sda part).

Sorry about your problem, but I appreciate the question: it led me to
<http://www.cromwell-intl.com/unix/linux-kernel-details.html>, a fair bit
of which was quite familiar, and other bits weren't. For example, cat
/proc/partitions might give you a serious bit of the information you're
looking for.

Hope that helps.

       mark