If it is not an HD box then it will be an IDE/PATA drive and if the box was never self upgraded with a larger HDD then it will be either a 40GB or 160GB depending on model of Sky+ box. What do you want to attach the HDD in a cradle to.
Thanks for all your help. I was just trying to get a cheap expansion drive for extra external memory. As you say hardly seems worth it... thanks again for your quick responses.