Group Discount Cannot Update 'ID'

[SolSearcher]

I am trying to break my products down to Discount Groups.
I followed the Actinic help file and created a product group and then added some products, I then applied a discount to that group.

When I selected one of the group products from the content tree I could see that the Product Group field was blank, I was expecting it to be populated with the Group Name, when I selected the drop down menu I could see the correct group so I selected it and the discount that I had created for that group appeared below.

Happy Days! Not. When I selected Apply changes the following error message appeared.

Actinic failed to save the product data in the external table. The error message reported was:
Cannot update 'ID' field not updateable.

I have searched the forum and the Actinc help file but cannot find anybody with the same problem.

[SolSearcher]

Ok, I've found the problem.

I have an external database linked to Actinic, it allows me to share stock levels over two sites.

If I disconnect the link to the external stock database, I can set up product groups, as soon as I reconnect the product groups loose the products associated with that group.