Comment # 5 on bug 960517 from James Carter

@Jiri, @Michael, thanks very much for the explanation.  I changed the
drive to (hd0), which gets the correct drive on all my work and home
machines.  I restored the saved image of v13.1 on my virtual machine,
edited /etc/default/grub_installdevice, and was able to upgrade it
and survive the reboot afterward.  

Some people boot off a disc other than the first, or a USB stick, or
a tamper-resistant CD.  I had a machine once with four discs that would
enumerate them in a random order.  In that case a device path like
/dev/disk/by-id/ata-ST9750420AS_5WS4F4QQ is more reliable.  

The reason I switched to the root partition's boot sector was, I noticed
that the installer's default was that way, and I just left the default
in place.  Bad move.  

I've set the bug's status to resolved.