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Date: Wed, 31 Dec 2008 01:27:22 +1100 (EST)
From: Ian Smith <smithi@nimnet.asn.au>
To: David Scialom <dscialom@gmail.com>
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Cc: freebsd-questions@freebsd.org
Subject: Re: Unable to modify sysid with Fdisk
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In freebsd-questions Digest, Vol 244, Issue 1, Message 6
On Sun, 28 Dec 2008 17:45:52 +0100 David Scialom <dscialom@gmail.com> wrote:
 > Hello,
 > 
 > It seems that it  is impossible to modify the sysid with fdisk since FreeBSD
 > 6.2. I am actually using FreeBSD7.0.
 > When I want to modify my the sysid from 165(ufs) to 12(Fat32) i get the
 > message "Geom not found: da0" and no change is made: da0 is stock with sysid
 > = 165. The detail are provided below. I tried also to do the same from the
 > install FreeBSD CD without success.
 > 
 > As someone a solution ?

David, I've skimmed through this thread so far, but I didn't notice 
anyone suggest, well, not doing it that way.

You want to change the MBR slice type of da0 from ufs to doswin32.  Is 
that to put an msdosfs filesystem on it?  Or is it already formatted as 
one of those?  Is it a 1GB USB memory stick?  Inserted but not mounted?

Try just deleting the existing ufs slice (assuming it's on a disk not in 
use) then readd that same size slice (here, whole disk) as type 12.  
That shouldn't touch any slice's data at all, just the MBR (and you've 
already dd'd the whole GB raw da0 disk to backup just in case ..)

I've used the fdisk wrapper in sysinstall to slice usb stick disks, 
sometimes with both msdos and ufs slices, just because I prefer that 
interface rather than maybe miscalculating an offset or size .. ymmv.

And from the darkside, FDISK x: /MBR probably still works :)

HTH, Ian