gpart and mbr give "no operating system" message at boot.

[markham breitbach]

I am trying to partition a disk to be used as the primary boot disk for a FreeBSD 8.3
installation using gpart to install an MBR partition.

The system is an existing FreeBSD 5.2.1 system at a remote location (ie impossible to boot
from CD/netboot/etc), but has no data of value.  To do this I am copying /boot and
mfsroot.gz from an mfsbsd iso image to boot to an MFS live system so I can wipe the drive
and do a clean install of 8.3.  After booting to the MFS I do this:

dd if=/dev/zero of=/dev/ad2 bs=1m count=1
gpart create -s mbr ad2
gpart add -b63 -t freebsd ad2
gpart create -s bsd ad2s1
gpart add -i1 -s 1g -t freebsd-ufs ad2s1
gpart add -i2 -s 1g -t freebsd-swap ad2s1
gpart add -i4 -s 2g -t freebsd-ufs ad2s1
gpart add -i5 -s 1g -t freebsd-ufs ad2s1
gpart add -i6 -t freebsd-ufs ad2s1
gpart set -a active -i 1 ad2
gpart bootcode -b /boot/mbr ad2
newfs /dev/ad2s1a
newfs -U /dev/ad2s1d
newfs -U /dev/ad2s1e
newfs -U /dev/ad2s1f

followed by a sysinstall and some configuration.  When I reboot I get a message that says
"Operating system not found" and the system hangs.

If I follow the same procedure but create a gpt partition it works swimmingly.  I am OK
with using a gpt partition if needed, but for the sake of curiosity I would like to know
why I can't make the MBR partition partition work.  Am I missing something?