Can't drill into exterior walls for screw anchors. Why not?
Hitting something super-hard means don't drill there.

Normally pipes and electrical cables are run far from the edge and close to the center of studs.   This is so you can safely drill 1-1/2" onto any wall's skin surface to install screws, anchors, etc. 

When it is necessary to place electrical wires or pipes just under the skin surface, they are required to put steel plates there as guards, to prevent someone from drilling onto the pipe or electrical wire. 

The idea is the person drilling will hit an unexpectedly hard surface, and think "this is probably one of those".