GPT/ZFS/USB mountroot prompt

[Carl Chave]

I followed a gpt/zfs on USB stick guide for putting a base 8.1-RELEASE
amd64 onto a 4GB sandisk USB memory stick.  All went fairly well and
the system will boot but fails to mount the root file system and dumps
me a the mountroot> prompt.  Entering zfs:zrootusb at the prompt works
and the system finishes booting.

In /boot/loader.conf I've got:
zfs_load="YES"
vfs.root.mountfrom="zfs:zrootusb"

zrootusb mountpoint is set to legacy and /etc/fstab exists but is
empty, per the guide.

Any ideas?

Thanks,
Carl