When you set up the Sky box, the options for Instant Rewind are:
OFF - 15 MINUTES - 30 MINUTES - 60 MINUTES
Depending on your choice, the Sky box will "reserve" space on the disk for that setting. I have a sneaky suspicion that it always reserves 60 minutes of space, even when it's set to a lower number, or OFF, but that's only a suspicion. Otherwise, it could have trouble if you have a 99.5% full disk with instant rewind OFF, and then you change it to 60 minutes.