ATA100 cables are very easy to split if you use this trick. Take the blade out of your utility knife or get a razor blade. Now, stick it in the "groove" between two wires, and PRESS DOWN, don't cut. You have a perfect starting point. Do this at every grove you want to cut. Then rip the cable pulling one end up and one end down. ATA100 cables are a BITCH to cut if you actuially have to cut them, but by using the "pressing down" method, your almost gaurenteed not to screw up.
-Chu