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impossible to restore MBR using dd


Cygwin version: 1.5.12
Windows XP Professional: Ver 5.1 Build 2600 Service Pack 2

I am trying to use Cygwin's dd command as a post Windows XP install command to restore our custom GRUB to the MBR after an unattended installation.
For safety reasons, I would like to restore only the first 446 bytes of the MBR to keep the existing partition table.

The command line to use would normally be:
   dd if=boot.MBR of=/dev/sda bs=446 count=1
-->    dd: writing '/dev/sda': No space left on device
         1+0 records out
         0+0 records in
(I also tried with bs=512 and I get the same output)

After reading:
I decided to try the following:
   mount -s -b -f //./physicaldrive0 /dev/hd00
   dd if=boot.MBR of=/dev/hd00 bs=446 count=1
-->   dd: opening '/dev/hd00': Invalid argument
(even if I change the device /dev/xxx name, I get the same output)

I even tried directly:
   dd if=boot.MBR of=//./physicaldrive0 bs=446 count=1
-->   dd: opening '//./physicaldrive0': Invalid argument

After so many unefficient tries, I decided to read from it, just to see:
   dd if=//./physicaldrive0 bs=446 count=1
-->   dd: reading '//./physicaldrive0': Is a directory
         0+0 records out
         0+0 records in

So here is the:
QUESTION1: Does anyone know how to write into MBR from Windows XP using Cygwin's dd ?
QUESTION2: I am doing somathing wrong since dd sees //./physicaldrive0 as a directory ?

Any help would be appreciated.


PS: If I do only:
   dd if=//./physicaldrive0
--> dd is wrinting things on the command line.

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