I have updated the OP with a complete, working NetBSD installation guide for Allwinner devices like the BPi that has been extended to cover the full process of transferring the root fs from SD card to a SATA drive in detail.
The origiinal guide that this was based on is
https://wiki.netbsd.org/ports/evbarm...nner/#index1h2
I'm going to submit a PR to hopefully get that official guide updated to cover these extra steps.