It's not like you could just plug in an external drive, and using any cloud service would be terribly expensive. Blu-Ray discs can hold a lot of data, but that's a lot of time (and money) spent burning discs that you likely will never need. Tape drives are another possibility, but are they right for this kind of problem? I don' t know. There might be something else out there, but I still have no feasible solution.
Lets start from the top: You *can* plug in an external drive, it's called a complete hardware duplicate of your array (or perhaps for space/cost consideration, a single disk based copy held offline and synced regularly). Not hard and not terribly expensive (i would go with this solution personally). Cloud? Yep the bandwidth and storage even on something like Amazon Glacier would be prohibitive to all but the most financially independent geeks. Bluray doesnt hold enough (even at 50gb/disc you need 400 of them, groan). So, tapes? You bet your ass tapes are designed to do exactly this task, why do you think they are still in use? You can get individual tapes at 1/1.5TB, but for a one man operation they are probably going to cost you more than the first solution (offline spinning disks) and they are a pain to manage properly.
Now what is this doing on ask slashdot? A pencil, some scratch paper, and 15 minutes between amazon.com and newegg.com would tell you the prices of every solution. Oh, right, they need a chance to tee up some targeted ads for Carbonite, Mozy, Crashplan, etc.