Diagnostic Repair Drones. These are like mechanical microbes racing through Moya defending it and repairing it. A constantly moving maintenance crew, it's significant that they are a finite resource. Unlike Moya, unlike the Transport Pods, DRDs are purely mechanical and can't be easily made in flight. They were probably installed at the same time as Pilot. Most similar to: Scutters in Red Dwarf; Huey, Dewey and Louis in Silent Running; R2D2 in Star Wars.