MBR blown away

Jerry McAllister jerrymc at clunix.cl.msu.edu
Sat Feb 11 07:32:31 PST 2006

> I need help.
> I added a slice to a single hard drive dual-boot (windows) system and now
> I guess that scrambled my MBR.  I get three options from the FreeBSD (5.4)
> boot manager:
> 1. DOS
> 2. FreeBSD
> 3. FreeBSD
> I can boot to FreeBSD (the new slice is fine) by choosing option 3 but the
> windows/dos option is fried.

The MBR itself looks OK.   According to that piece of menu you posted, you 
just added another bootable slice.  So, there are now two bootable FreeBSD 
slices and one bootable Microsloth slice.   

Are you saying that the MS slice will no longer boot if you select '1' from
the menu?    If that is the case, it is not the MBR that was messed up.  
It is something in the MS slice - probably their boot sector.   I don't even 
pretend to know how MS sets up theirs if it is any different from FreeBSD.

But, the MBR is doing what it is supposed to do.  It discovers all the
bootable slices and makes a menu and transfers control to the selected
slice.  What happens after that is not the problem of the MBR.

That may be bad news, I suppose.  It might be easier to fix the MBR
than the MS slice boot code if it is actually messed up.

It might be as simple as you managed to mark the MS slice as not bootable
in some way, but in that case, I wouldn't expect the MBR to be able to
see that slice and put it in the menu as bootable.

Did you use some utility to shrink the original two slices to fit in
the new one?  Or was there already unused space (previously unallocated)
that you were using?    Maybe the utility you used to shrink the other
slices messed something up.    You might need to go back to it and
check it out.

Was the MS slice an NTFS type file system?   Many of the free utilities
for resizing slices do not work properly on NTFS systems.   So, it is 
possible, in that case, that the MS slice was not shrunk properly and
so it got trashed at that stage.

Just some thing to consider.
Good luck,


> My current strategy is to use boot0cfg:
> # boot0cfg -B
> But I'm a little squeemish.  I don't want to be locked out of FreeBSD (I
> barely use Windows but I still would like it back for Visio).  Any
> guidance?

As per my comments above, I don't think rewriting the MBR will help any.
> --
> Peter
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