Just had a thought about using dd... won't be much good for your purposes. Yes it will clone the whole disk from the old to the new, but it will keep the partition the same size as the original. Speel is right - you would need to know what format the disk is in so you can manually reproduce the partition layout (assuming it's in a non-proprietary format).
Have you considered the possibility that the device will format the disk for you? Some Sky+ boxes do this. You just put the new disk in and the box detects that the disk doesn't contain the right data and formats it for you. There's no risk in trying it.
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moto