If your hard drive previously had a GPT boot loader this can happen. It has something to do with the GPT boot loader using the first 2 sectors. NetBSD boot loader uses 1 sector and only writes to the first sector. I've had consistent success wiping out the GPT data using gdisk as described
here. gdisk is available with gparted, PartedMagic, System Rescue CD and as a separate Debian package.