> I notice I cannot mount the newly created partition as /

Since you gave "ra0*" as the name of the root-partition, the swap-partition
(ra0b) is mounted on /, thus you can't mount another partition on /.

> I have tried to copy boot and gennetbsd to ra0a using the following which it does not complain about:-
> 
> # cp boot ra0a
> # cp netbsd ra0a

You just copied these files within the same directory to another name.
what you want to do is:

	# mount /dev/ra0a /mnt
	# cp boot /mnt
	# cp netbsd /mnt
	# mkdir /mnt/usr
	...

> ( also tried #cp boot /dev/ra0a  etc.)

You can't copy to the device directly. You first need to mount the
device into your filesystem. Then you can access the mounted device's
filesystem and copy files into it.

> When  I try to reboot the system with
> 
> >>> B DUA0
> 
> 2..1..0
> 
> I get the first line :
> 
> howto...
> 
> then it says :-
> boot failed, errno 21 (Unknown error: code 12)

bootblocks can't find the /boot program since it's not yet copied
into the filesystem in ra0a.

Ciao,
	bertram