OK, that helped, but not in the way you might expect!

After comparing the mbr contents with /usr/mdec/mbr and finding only
a small difference (looks like a list of sectors or something), I 
decided to reboot once again to get the exact error message.  And I
noticed that the BIOS was booting from HDD-1 (wd1) rather than HDD-0!

I fixed the BIOS setting, did "fdisk -c" and another "installboot"
and presto - it works now!

Thanks for the clue.

On Sat, 9 Apr 2005, David Laight wrote:

> First, I'd check that you have the correct code in sector 0.
> eg run 'hexdump -C -n512 /dev/rwd0d' the output should match that
> of the installed file from /use/mdec/mbr*.
> If it is wrong, 'fdisk -c mbr' should try to update it.
>
> Some of the 'Not a system disk' error messages are actually output
> by code on the disk itself.
>
> If the bios is refusing to believe that the mbr is valid it gets a
> bit harder....

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