Burroughs also supplied the specialized disk drives, which featured a separate stationary head for every track and could offer speeds up to 500 Mbit/s and stored about 80 MB per 36" disk. They would also provide a Burroughs B6500 mainframe to act as a front-end controller, loading data from secondary storage and performing other housekeeping tasks. Connected to the B6500 was a 3rd party laser optical recording medium, a write-once system that stored up to 1 Tbit on thin metal film coated on a strip of polyester sheet carried by a rotating drum. Construction of the new design began at Burroughs' Great Valley Lab. At the time, it was estimated the machine would be delivered in early 1970.