I would like to have the www directory to be located on hdb2. This is how I do, I bet there are better ways, but this works fine for me. In the boot-section of /rc/rc_user I do this:
mkdir /mnt/disk2 chmod 755 /mnt/disk2
ln -nfs /mnt/disk2/www /www ln -nfs /mnt/disk2/www /mnt/bootdev/router/www
mount -t ext2 /dev/hdb2 /mnt/disk2
Maybe I should have mounted the disk earlier in the process, but this works fine.
Hope this helps