Thanks but this didn't really help.
I can buy the drives at a dozen places but none of them seem to know what connection is correct (SATA etc) and that includes the apple store. It was the store who told me buy the superdrive and all my problems would be over. Wrong! It isn't recognised by imac as it's confifured for air and mini mac. Only way to make it work is as i said in earlier post. (Would you be happy buying a car you were told would start and then discover you have to open the bottome and fiddle to make it go?)
The apple store I would not even consider taking it to. Days without the pc, over priced and don't have a good reputation for actually fixing things!