"boot0cfg: read /dev/ad1: Input/output error" using nanobsd

[Luke Dean]

I'm running FreeBSD 7.0 on a soekris 4801 with an image built by nanobsd. 
It's a small piece of headless hardware that boots from a compact flash 
drive - no moving parts.

I wanted to update the operating system to 7.1 and install some more 
packages on it, so I built a new image with nanobsd and uploaded it to the 
second partition using nanobsd's updatep2 tool.  The new image mounts 
fine, but the last line of updatep2, "boot0cfg -s 2 -v ${NANO_DRIVE}" 
fails with the message I've quoted in the subject line.

The machine boots fine, but I can't provoke any kind of response from 
boot0cfg except for input/output errors.  I'd like to make the machine 
start booting from the second slice.

Any ideas?