Segue dica para instalar o OpenBSD 5.4 com dois discos em RAID 1 via software (softraid).

(I)nstall, (U)pgrade or (S)hell? s
# cd /dev
# sh /dev/MAKEDEV sd1 sd2
# fdisk -iy sd0
Writing MBR at offset 0.
# fdisk -iy sd1
Writing MBR at offset 0.
# disklabel -E sd0
Label editor (enter ‘?’ for help at any prompt)
> a a
offset: [64] [ENTER]
size: [41929586] [ENTER]
FS type: [4.2BSD] RAID
> q
Write new label?: [y] y
# cd /tmp
# disklabel sd0 > protofile
# disklabel -R sd1 protofile
# bioctl -c 1 -l /dev/sd0a,/dev/sd1a softraid0
sd2 at scscibus2 targ 1 lun 0: <OPENBSD, SR RAID 1, 005> SCSI2 0/direct fixed
sd2: 20473MB, 512 bytes/sector, 41929058 sectors
softraid0: RAID 1 volume attached as sd2
# dd if=/dev/zero of=/dev/rsd2c bs=1m count=1
1+0 records in
1+0 records out
1048576 bytes transferred in 0.009 secs (111337439 bytes/sec)
# exit

O instalador voltará para a tela inicial. Escolha a opção (I)nstall e proceda normalmente com a instalação. Lembre-se de realizar a instalação no disco sd2 (volume RAID criado pelo softraid).

Referência:
http://spiritedblowfish.wordpress.com/2012/07/19/installing-openbsd-5-1-amd64-using-softraid/