You cannot let evidence get destroyed. If you know, or should know, or even if you should have had an inkling that data that could be subject to discovery request is being destroyed and you don't stop it, it will go badly for you at trial. I suggest you go out and buy more storage space in your hypothetical. Think of it this way.. If you knew that there was a box of evidence headed to the paper shredder because it was in the closet with the rest of the "to be shredded" boxes and you didn't go find that box, they'd come after you for letting evidence get destroyed. A disk drive or backup tape is no different.
You see, the opposing lawyers will argue that you knew evidence was being shredded and did nothing to stop it, so you allowed damming evidence to get destroyed, KNOWINGLY. Which, they will argue, means you knew you where guilty and where trying to cover it up. So do what you want, but any competent lawyer will tell you that you'd better do everything in your power to preserve data once you become aware of a legal action that might involve said data. Now if you are a criminal, then you are going to do what you want anyway and why are we having this conversation?