Heathrow (1985 approx.)
I seem to remember each unit was 20Mb, consisting of 4 x 5 MB platterns. They were fixed head (one per track) drives. (Possibly only 2.5 Mb per plattern giving 10Mb with two cabs being 20Mb - not 100% sure). The spare disk units were bought by Bob Bean from a scrap merchant near Kew Gardens to keep the system running with spares (thats 1980's Air Traffic systems for you). These were part of the AFTN (Aeronautical Fixed Telecommunication Network) message switch.