The Cargo Drone is an auto-piloted ship that ferries unrefined minerals collected by a Miner to the Battlecruiser.
Only Miners can create Cargo Drones, using the "Return Minerals" ability. Once the skill is used all the collected unrefined minerals in the Miners inventory is transferred to the Drone, and the Drone heads straight towards your Battlecruiser. When the Cargo Drone reaches your Battlecruiser it drops all its inventory at the cargo drop off point.
Cargo Drones can be intercepted and destroyed by the enemy resulting in a loss of all the collected minerals on board.
Cargo Drones hull can be upgraded by researching tier 1 economy.
Cargo Drones AI don't account for map warp.
Ships cannot collide with a Cargo Drone.

-info add: cargo drone engine

Drop off point

When the Cargo Drone reaches your Battlecruiser it will deposite the minerals inside its inventory at the bottom of the Hangar on the Battlecruiser, this is the Cargo Drone drop off point for unrefined minerals.
For more information on the Battlecruiser Layout see this.

Cargo Drone