My plan is to have triple backup...original data, single redundancy under my desk with the 1TB drive mentioned above, then a third 1TB drive for off-site redundancy. That third 1TB drive will come to work with me where I'll keep it locked up in my desk drawer. Maybe once a month I'll take it home to refresh my data and then bring it back to work...or maybe at that point I'll rotate with one of the active drives so that all of my drives get about the same amount of use.
I agree with everything tonedeaf and Brian Skalinder wrote, and I use a technique similar to what tonedeaf mentioned above. I have a desktop machine with 3TB storage, a backup server with 3TB active in RAID5, and a 2TB external disk (2 x 1TB) that gets stored at work. I bring the external disk home every week or two and sync it against the backup server. All of my media are stored on active disk. This method uses a lot of disks but the price per GB is still pretty cheap and this is a lot more convenient than dealing with optical disks, IMO. The key to this approach is redundancy, as the other guys said.
The time will come when the 2TB external drive isn't big enough and I'll have to add another external disk. But by then I think we'll have other options. Heck, you can already buy 2TB internal drives - amazing