dd: /dev/rsd1c: device not configured

Hi,
I'm trying to install 7.0 in a virtual box machine using full disk
encryption, following
<https://www.openbsd.org/faq/faq14.html#softraid>. I've done it on
real hardware without a problem, but I'm not understanding the error
in the virtual box machine. In particular, I cannot copy random data
on the disk before doing the effective encryption.
This is what I do, after entering the shell at the very first prompt:


# sysctl hw.disknames
hw.disknames=wd0:,cd0:,rd0:7c72fe60b4e2338d

# ls /dev/rsd*c
/dev/rsd0c

Uhm, why is sd0 there and does not appear in the hw.disknames?
However, I tried to configure the sd1 device:

# cd /dev
# sh MAKEDEV sd1
# dd if=/dev/urandom of=/dev/rsd1c bs=1m
dd: /dev/rsd1c: device not configured

# ls /dev/rsd1c
/dev/rsd1c

What am I missing here?

Thnaks,
Luca