I know the groove. It's there to do a couple of things....
1. to stop the pads glazing,
2. to vent any gas buildup under the pads which causes brake fade.
It's this second one which I assume would determine the direction of the disc. Can't seem to find the words to explain why, but so the grooves can scoop the airflow properly I'm thinking the leading edge of said groove must be at the outer edge of the disc.
see what i'm on about?