Most DVR boxes use a proprietary format for recording - unlikely to work for slotting into a PC or Mac, Linux may be a possibility. You may though have a better chance of swapping into the other box, but beware of it 'assuming' a new drive and wanting to format it...
It's a while since I've looked, but it may be worth asking/ browsing on the DVR sites - e.g. digital spy or avforums?