During the mission to Virmire to stop Saren, Shepard and the squad have to fight through Drones in order to get to the base. Even after infiltrating the base, the geth deploy more Drones to keep Shepard from destroying the base.
Geth Assault Drones are armed with a Mass Accelerator Machine gun that like the Advanced Assault Drone, can rip right through your shields and health. These Drones are typically found with large concentrations of geth troops, usually in open areas. Because the Drones like to flank you, keep on the move, and avoid getting to attached to the cover that you are behind. Because they are also synthetic enemies, using tech powers like overload and sabotage are very effective against them, especially in large groups.
Geth Assault Drones are also flying enemies, note that most biotic powers won't work against them. Only powers such as Warp have any effect, so if you are going up against these enemies, make sure to bring along a tech savvy squadmate.
Because quite a few of these enemies can be encountered while in the Mako, however exiting the Mako to combat them might be a good idea. A direct hit from the Mako's main gun can to a lot of damage, but considering it is hard to get an accurate shot with the main gun, using the Mako's machine gun might be a better option. The Mako's shields can only stand up to so many hits from these flying enemies, so keep on the move, and if there are other enemies nearby, watch your shields and retreat if necessary.