Booting a USB hard drive

[bob prohaska]

Are there any descriptions floating around for booting an
rpi2 from a usb hard disk? I thought it would be a simple
matter of loading a crochet image on both microSD card and
hard disk, then adjusting the boot config on the microSD card
to point at the hard disk.

In fact it does work, but only by stalling the boot process
at the mountroot prompt long enough for the hard disk to
be (re) discovered, whence issuing the ufs:da0s2a command
results in a clean boot of the hard disk, with the microSD
card left unmounted.  

Apologies if this is a premature request; there are many
discriptions of how to boot Raspbian from USB, but I could
not find any guidance on how to do it with FreeBSD. The 
small success thus far seems to have been a matter of luck. 

Thanks for reading, and any guidance.

bob prohaska