[Fwd: Re: impossible to restore MBR using dd]

[Matthias Bertschy]

-------- Original Message --------
Subject: 	Re: impossible to restore MBR using dd
Date: 	Tue, 03 May 2005 19:10:20 +0200
From: 	Matthias Bertschy <[EMAIL PROTECTED]>
To: 	Eric Blake <[EMAIL PROTECTED]>
References: 
<[EMAIL PROTECTED]> 


Hello Blake,
Eric Blake wrote:
Hello,
Cygwin version: 1.5.12
   

Can't you upgrade?  coreutils-5.3.0-5 is out, and requires a newer version 
of cygwin.  Perhaps with the latest versions you may pick up a bug fix along 
the way that helps solve your problem.  Also, see 
http://cygwin.com/problems.html to give us more information so we can help you 
better.
 

As suggested, I have upgraded to the latest cygwin. Unfortunately, it 
doesn't change the results :-(

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)
   

Hmm - writing just 446 bytes to raw devices is not very safe.  /dev/sda is 
a block-special device for a reason, and writing anything more or less than a 
full block at a time on a block-special device is not guaranteed to work.  
Maybe that is your problem?  Also, according to `stat /dev/sda', the IO block 
size of an NTFS raw device is 1024, so even bs=512 may be equally dangerous.
 

Maybe I should recall you that on Linux, we can safely write 446 bytes 
blocks to a raw device. Indeed, the command dd if=boot.MBR of=/dev/sda 
bs=446 count=1 works under Linux.
I also don't understand why you are using NTSF raw devices to access the 
MBR... after all the MBR isn't formatted at all !

I think I will fill a bug report for the dd command under Cygwin. Could 
you point me where to do it?
(or maybe for Cygwin's ability to access raw devices)

 

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

That's to be expected.  The of= flag is necessary to tell dd to put the output in the named (device) file, rather than on stdout.
 

I know :-) that was just to point out that without any bs parameter, the 
dd command works.

--
Eric Blake
cygwin coreutils maintainer
 

Matthias

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