I'm not sure if that's true mate just on the assumption that the sly boxes have a 500gb hard drive and can fit a few hours of programming but I might be wrong.
That's why I'm thinking 2tb will be ample.
Long winded but I managed to find the sort of drives that's in the sky boxes,bare in mind I probably have about 10 episodes of motd on the hdd as well as other things so there is 10+ hours there,so I think going buy the bottom drive a 2tb hdd will be more than ample.
SD Boxes
PVR1 - Pace BSKYB3000 (9F 20xx) - 40GB (0GB/40GB System/User)
PVR2 - Pace BSKYB3100 (9F 21xx) - 40GB (0GB/40GB System/User)
PVR2 - Amstrad DRX-180 (4F 20xx) - 40GB (0GB/40GB System/User)
PVR2 - Pace BSKYB3100 (9F 21xx) - 160GB (80GB/80GB System/User)
PVR2 - Thompson DSI6210 (4E 20xx) - 160GB (80GB/80GB System/User)
PVR3 - Pace TDS470N (9F 22xx) - 160GB (80GB/80GB System/User)
PVR3 - Pace TD570N (9F 23xx) - 160GB (80GB/80GB System/User)
PVR3 - Amstrad DRX-280 (4F 21xx) - 160GB (80GB/80GB System/User)
PVR3 - Thompson DSI8210 (4E 21xx) - 160GB (80GB/80GB System/User)
HD Boxes
PVR4 - Thompson DSI8215 (4E 30xx) - 300GB (160GB/160GB System/User) *No longer updated, stuck on OpenTV FW
PVR4 - Pace TDS850NB (9F 30xx) - 300GB (160GB/160GB System/User)
PVR4 - Samsung HDSKY (97 30xx) - 300GB (160GB/160GB System/User)
PVR4 - Amstrad DRX780 (4F 30xx) - 320GB (180GB/180GB System/User)
PVR5 - Samsung HDSKY 500 (97 30xx?) - 500GB (250GB/250GB System/User)
PVR5 - Amstrad/Sky DRX890 (4F [3100-314F) - 500GB (250GB/250GB System/User) * Still in production
PVR6 - Amstrad/Sky DRX895 (4F [3150-31FF]) - 1.5TB (500GB/1000GB System/User)
PVR7 - Sky DRX895 (4F [3150-31FF]) - 2TB (500GB/1500GB System/User) * Still in production