i had the same problem the hdd lights up blue but the MBPR doesnt read it at all. i read around and found a temporary way around.
basically if your original cable looked like this:
you can instead use a micro usb cable(like the one kindle fire/blackberry uses)and connect to the computer with that and it was instantly recognized problem is that it uses USB2.0 speed also the light is white not blue
yeah so just updated to mountain lion and this drive still doesnt work on my Mac computer using USB3.0 cable.
i'm pretty sure this issue is with Toshiba so just to warn everyone out there DO NOT BUY TOSHIBA CANVIO 3.0 hdd it wont work on the new macs