continued....
Drives over 300GB have been tested but do NOT work on any standard definition Sky+ boxes tested so far, unless their capacity is limited using the drive manufacturers tools to around 270GB. This seems to be a fundamental limit of the Full System Reset (FSR) function of Sky+ software. The way around this is to copy over all your recordings from a smaller drive using the Copy+ software and your PC. Use of Copy+ in this way has been tested by a very small number of users and shown to work for drives up to 500GB. However please remember that this workaround is still experimental, and if you ever need to do a FSR in the future the box will not be able to correctly reformat the drive. The only option will be to return the drive to your PC and use Copy+ once again.
Sky HD: The maximum size drive that has been tested appears to be 500GB, and this works correctly, even for a FSR. Please remember that 140GB of this space is still reserved by Sky, which will leave you with 360GB for your personal recordings.
http://www.digitalspy.co.uk/forums/showthread. php?t=156344