If you use FreeBSD to create the installation disk:
https://www.freebsd.org/doc/en_US.IS...stall-pre.html
With OpenBSD:
https://www.openbsd.org/faq/faq4.html#MkInsMedia
As Keith said, you should use the whole disk (da0 in your example). So in both cases you will lose all data on the target disk.