JimmyH: It's like modifying a car.
If I rent you a car, then I would be pretty angry (and justifiably so) if it was returned with a new paint scheme, a wide exhaust, and a souped up engine with nitrous fitted. I would come after you legally.
Saying that you were just making it better, or that it was justifed because the rental cost I charged for a more powerful car was outrageous, wouldn't help you in court.
You need to either:
(i) put up with it.
(ii) cancel your subscription and walk away
(iii) try find a way of doing what you want without damaging someone else's property
Your analogy is not relevant several ways.
There is no cost to sky when the mysky box is returned as it is in original condition but in your analogy there is a cost to restore the car to original condition (incidentally, I'd agree the renter must restore the car or whatever to original condition).
A better analogy would be that someone puts snow tyres on the car as it is snowing. Then, they refit the original tyres before returning the car. You are not out of pocket, the car is exactly the same as you rented it out.
ie, you take the sky box, insert a larger hdd, then, refit the original hdd when returning. There is no cost to sky (other than the $600 they may have charged for an upgrade, but, they would never receive this anyway as people simply wouldn't pay that much to upgrade).