2007/09/18 の日記 を見て、

h97@makoto 08:06:44/230204(~)% sudo vndconfig /dev/vnd0c 
    /home/makoto/Downloads/NetBSD-9.0-amd64.iso
vndconfig: /dev/vnd0c: opendisk: Device not configured

h97@makoto 08:06:47/230204(~)% ls /dev/vnd0c
/dev/vnd0c

h97@makoto 08:07:02/230204(~)% sudo vnconfig /dev/vnd0c 
    /home/makoto/Downloads/NetBSD-9.0-amd64.iso 
vnconfig: /dev/vnd0c: opendisk: Device not configured

何か変った ? man(1) vndconfig して

vndconfig -l
vnd0: not in use
vnd1: not in use
vnd2: not in use
vnd3: not in use

vnd0c でなくて vnd0 なら OK

sudo vndconfig -c /dev/vnd0 /home/makoto/Downloads/NetBSD-9.0-amd64.iso
sudo mount -t cd9660  /dev/vnd0d  /mnt2