I've been playing with a 4-axis controller, and I had assumed that it would have the equivalent of "active high" or somesuch on the error signals from the servo drives, so a disconnected driver would show as OK. Not so! However, it does seem that the 4th axis has been disabled so it does not throw an error. Maybe SZGH did this in their initial configuration (although they did make one mistake - the estop input from the MPG was configured the wrong way round so that you needed to hit the estop to make it work! Soon fixed).

One slight frustration that might be relevant to Chaz as well. There is an encoder output from the servo, and the controller has encoder inputs available. Presumably these are for closed-loop operation although I think that you can configure the coordinate readouts to use them. However, the provided cables, excellent as they appear to be in all other ways, make no provision for connecting encoder output to controller input. My mate who owns the machine would have liked the ability to use the controller readouts as a DRO for manual operation with the table handwheels. It looks at first sight as if you would need to hack into the cables or connectors to provide this capability.