Thanks for both your replies.
I disconnected the DNS-320 box from the power, reconnected, switch on, magic, the blue drive light came on and the drive became accessible through Firefox.
Tried formatting the new drive but the process stopped at 14%. Shut every thing down, restart, same again, 14%. Shut off DNS-320, remove and reinsert disk, back to original problem with no disk being registered either in Firefox or blue light on NAS box. Shut down, etc, blue disk light on, attempting format but no progress beyond 0%. Now cannot switch off NAS box from front panel so disconnect power, restart, disk light on but does not stop flashing. Firefox shows no disk.
Enough for one morning.

The DNS 320 is about two years old, from Amazon, and has performed fine until now.
I will try for a replacement under warranty, if still valid. Checked, only a one year warranty.
Any further suggestions? I may buy the D-Link DNS 320L.