What can happen if the starter drive is not removed from the start?

If your starter fails to dis-engage after start you should stop the engine immediately. Serious damage will occur to the starter and more importantly the starter ring gear around the engine flywheel. That would be an expensive fix, which involves removing the engine or gearbox or (on some cars) both. The other problem is the possibility of an engine fire. If the engine is allowed to turn the starter at high speed the starter will begin to act like a generator and this uncontrolled electrical power generation can lead to an electrical overload and electrical fire, taking with it all voltage sensitive electrical components on line at the time.

Don't risk it. If your starter will not dis-engage, remove it and get it repaired and or, (if fitted separately), check the main starter solenoid to see if it has dis-engaged.

If the starter drive is not removed from the start the engine will tent to drive the gear placing stress on the gear which will eventually wear off the gear.