Failing to get installboot to work, cross-device install -> ERR M amd64 6.6-stable

Hi,

I'm stuck at 6.6-stable, trying many times in the past to use newer
-currents is partially to blame for how I'm in a bad situation.

I'll try to be clear as I can.
After the error below, booting gives ERR M.

What I'm using.

One USB2 flash drive. Booting / off of it. It works fine, but old.
One USB2/3 spinning hard drive. 2TB. Never setup to boot from. Using for
storage. The a partition has the files to get a boot started.
One Laptop spinning HD. The a partition used to boot, has the files but
won't boot.

I also have a powered USB hub to get enough power for the spinning HD.

I have tried using fdisk on laptop and USB2/3 HD. Not helpful.
I have tried using installboot on both of those two, but I get cross
device install error, which I see is only in i386 installboot in src.
I am following installboot manual page. I also read all of the
recommended see also man pages.
I also burned an install CD and took it just far enough into install to
get access to laptop HD, mount /mnt/usr and try installboot from there.
Same cross device install error.

I'm unsure if I should try a fresh install (backing everything up is a
little weird the way I'm running off of three disks. I can do that.)
I would like to understand what is causing me to end up at ERR M.
I read what causes that error, but I don't know how to fix that.

Any advice appreciated.
Chris Bennett