My sincere apologies, I wrote the wrong thing yesterday. I am not using Execute Shell Script, I am using Execute Applescript!!! 
My macro is a little long so I have exported a short one that just does the actions we're interested in. When I run it, I get to the Growl, but the backup does not start. (The disk in question is mounted on my desktop)
Untitled.kmmacros (1.9 KB)
Untitled.tiff (113.9 KB)
I had a look at Execute Shell Script but in my KM there is no way to specify the input for it, so I would prefer to use Execute Applescript if possible.