here is some very good guides to replace the HDD's in sky+ and skyHD boxes.
upgrade HDD in sky+
upgrade HDD in skyHD
hope they help:)
Found these links while searching -
How to upgrade your Sky+HD's hard drive: Project: Supersizing your Sky+HD | News | TechRadar
Replace your SKY+ HD Hard drive
The second one then links off to guides depending on the manufacturer of your box.