Eu encontrei a resposta de Nick Holland na lista de discussão do OpenBSD:
So...today, you take a couple disks, zero the first 10MB, put a 1G boot
partition and make the rest RAID, then build a mirrored set, do your
testing, and call it done.
Tomorrow, you take the same disk, zero the first 10MB, put a 1GB boot
partition on it, and make the rest RAID, and intend to build a crypto
RAID partition on it. Except...Poof! your RAID1 chunk is baaack! Why?
Because you didn't touch the softraid data which is 1GB up the disk.