From Starfleet Commander
The Mine Droid is a type of droid assigned to the Mines to help speed up production. Other than Human Worker and Basic Androids, they are the only type of workers that can be assigned there. You are allowed one worker for a level 1 Mine, a second at level 3, and then one worker for every 3 levels. So for instance, a level 3 Ore Mine can have 2 workers, a level 6 can have 3, and so on.
Each Mining Droid increases the speed of production by a set amount.
The default Mining Droid gives you a 2% boost in productivity. Basic Androids provide a 4% productivity boost.