It *is* a simple hard drive - but the box will not allow recordings to be played unless the box has a valid satellite signal and a correctly-paired viewing card in the slot. This is to prevent people "loading up" a box with lots of subscribed programmes and then lending the box around to non-subscribers. Similarly if you just use the box as a freesat box (no sky subscription*) you don't get any of the recording/pause functions. If you want to watch sky programmes when there is no satellite signal you have to use the web-app (sky Go).
Actually all the above applies to the Sky+/SkyHD boxes - I don't know how much of it applies to the more internet-based SkyQ system.
PDR
* Sky will sell you a freesat card to use with surplus sky boxes if you ask them