Booting from USB on RPI3

[bob prohaska]

On Fri, Apr 24, 2020 at 08:32:54AM +1200, Jonathan Chen wrote:
> On Fri, 24 Apr 2020 at 08:22, Mark Millard <marklmi at yahoo.com> wrote:
> [...]
> > The RPi3 will not start to boot from a gpt partitioned
> > media. So picking gpt labeling as the example is somewhat
> > misleading for single-media booting. glabel based
> > labeling would be more realistic for the context.
> 
> The OP is attempting to boot off an external USB drive via loader.env.
> So it's the external drive's partitioning system that is of interest.

The USB drive on my system was written with dd using
 FreeBSD-13.0-CURRENT-arm64-aarch64-RPI3-20200409-r359731.img
It's been resized to fill the hard disk and given a swap partition.
Gpart reports
bob at www:~ % gpart show da0
=>      63  78140097  da0  MBR  (37G)
        63      2016       - free -  (1.0M)
      2079    102312    1  fat32lba  [active]  (50M)
    104391  78035769    2  freebsd  (37G)

gpart show da0s2
=>       0  78035769  da0s2  BSD  (37G)
         0        57         - free -  (29K)
        57  71303168      1  freebsd-ufs  (34G)
  71303225   6732544      2  freebsd-swap  (3.2G)

To my surprise, 
gpart show da0s1 reports
gpart: No such geom: da0s1.
even though there is a /dev/da0s1. I thought it would at least report
the size.
 
No attempt has been made to alter the msdos partition,
That does mean the u-boot on the USB drive isn't quite
the same as the u-boot on the microSD, which uses 
https://github.com/raspberrypi/firmware/raw/master/boot/bootcode.bin
and modified to turn on its serial console. Could that matter? Since
it appears the USB installation is getting all the way to the loader
prompt on USB I'm tempted to think the trouble's on the FreeBSD side
of the USB setup. 

Thanks to all for reading, and any suggestions. 

bob prohaska