First of all, i probably would trust DVD/bluRay rather than HDD for long term archival storage. HDD has electronic and mechanical components and may fail even if you just let it sit unused for years: moisture, magnetization, etc. You can soak a DVD in water and it'll still read. If you're paranoid, burn more copies.
With several TB of data it's seems crazy to burn it all in DVD/bluRay, but your application is very specific: video. Have you consider using higher compression for archival purpose only? If you're ripping to divx/mpeg4 now, switch to h.264 at a lower bit rate. If you're using h.264 now, use h.264 AVC, downscale the resolution, reduce I/b-frames, downmix AC3 to 128kbps stereo AAC, etc.
Even DVD will look like VHS to us in a few years, so will bluRay. I'd be willing to sacrifice a little quality to make archiving easier and smaller, when the truth is that by the time you need that backup copy it'll look like VHS whether it was native mpeg-4 or h.264 AVC because technology has processed 10x already.
How many DVDs do u have and how are you encoding them now? At 1GB a piece compressed you need 1000 DVD to fill a 1TB drive.
If I were you I'll compress the hell out of those movies if i want to keep and burn them on DVD or other cheap redundant offline media or upload them to the cloud, but keep the higher quality rip online in hdd for instant viewing. Majority of them are not gonna be the action packed blockbusters that need high resolution and bitrate. And for the blockbusters, use higher bitrates for them but still keep a lower bittate copy offline for storage.
If that all sounds too complicated, it is. It makes you ask yourself the tough question, what data is more important than the other? just how much quality do you really need from a backup copy? And how much money and time do you really want to invest in a bit-perfect copy for everything? If you want to take the easy way out then just mirror everything on RAID. And if even then you wanna make the argument that RAID is not a real backup, that's just digging a deeper rabbit hole for yourself.
For music I have a simple painless backup strategy, ie. none. My primary library is on one hdd. I sync my 2 iPod and iPhone with it. If the library hdd dies, I still have the music I have on my iPods, which contain the most important selections of what I like and listen to the most anyway. The rest I'll lose. And I'm fine with it. I might get a backup drive for my music someday, but it's not a priority for me to backup "everything".
Just that we live in a world of bit-perfect copies doesn't mean we have to make a bit-perfect copy of everything we have. Prioritize your movie library, some you can stand to lose, some you can keep a low-bit rate copy. Even for the important movies, chances are you'll buy the bluRay version in a few years, and then the 3d version, and so on. Their future redundancy is already built-in to you.
And lastly, if raid is not real backup, then neither is a hdd storage tower elsewhere in the house. It's still a bunch of spinning hdd. They WILL fail in 3-5 years. It's a statistical inevitability. If you buy 2TB for backups, that's extra 2TB for you to upgrade every 3-5 years.
Just remember, all these DVD quality video will look like VHS to you in 10 years, and you can stream most of them from NetFlix for $15/mo, TODAY, already cataloged for you, and some even with 1080p.