On Mon, Mar 22, 2010 at 04:41:39PM +0800, Uwe Dippel wrote:
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> Having done upgrades from 4.0 onwards, on a OpenBSD-only server
> (amd64), this time something must have gone wrong: Despite of the
> (remote, I have no physical access, via serial console) 'successful'
> upgrade (no error messages), when I was asked to reboot, I did, as
> always. Alas, it came up with
> 
> Attempting Boot From Floppy Drive (A:)
> Attempting Boot From CD-ROM
> Attempting Boot From Hard Drive (C:)
> Using drive 0, partition 3.
> Loading...
> ERR M

That is biosboot(8) telling you that it cannot find /boot, which is the
boot loader that prints the boot prompt and so on. Biosboot has to be
really small, so its error message are terse.

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> 
> on an HP ML350G4p.
> 
> From all I know it is a problem with the MBR.

As explained above, no, you likely moved around/corrupted /boot in a way
that doesn't work for biosboot.

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> What I'd really like to get, before I drive there and get access, is
> how to best solve this problem, and most straightforward. Talking
> about what went wrong can wait, since this is a production machine
> and should be back as soon as possible.

The procedure is straight forward and documented in installboot(8).

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> 
> Thanks in advance,
> 
> Uwe