i wrote:

>One possible cause is that your BIOS expects the entire first track
>(possibly even first cylinder?) of the disk to be left free for a boot
>manager. You could try booting the install disk, backing up your root
>partition, editing the BSD disklabel to align the start of the
>filesystem on your `a' partition to a track or cylinder boundary,
>and unpacking the backup.

Oops... you'd need to do a newfs *after* changing the BSD disklabel
offset of the `a' partition, but *before* unpacking the saved copy of
your root partition. You'd also have to rerun installboot after
restoring your 'a' partition.

Again, re-installing with the start of `a' aligned to a track boundary
may be simpler, if reinitializing the MBR with /sbin/fdisk and
installboot doesn't help.

But be sure to save your existing MBR setup first, before you start
experimenting.