Thanks. But you were too quick...
I had a brain wave... Perhaps that isn't the right word. But I started to wonder whether the SD drive in the Macbook pro was having issues. So I used an external Card reader. It stated that the card wasn't initialised. It was, but I didn't care, I told it to initialise it, and then format it. Once I did that, Stick saw the card. And what's more it continued to complete the process. I placed the card in the camera, and CHDK started. So, I think I am now ok. But I am a little concerned about that card now... But Stick is an excellent process. Makes the Mac process so much simpler.
Thank you for your help.