Have you looked into an LTO-5 system instead of a multi-drive backup? Store the tapes properly and they will work for several decades untouched. It's a bit expensive, but it should outlive further drives you'd have to buy in 4-5 years. I do a lot of filmmaking and video editing, and LTO seems to be the standard for data archival in most post houses. I don't personally own one, but that's only because I'm broke as fuck. Edit: Also, avoid Seagate in the future, their drives are known to fail faster than other brands.
Avoid Seagate is good advice. There was that whole Backblaze analysis that got a bunch of people worked up on hard drive reliability http://blog.backblaze.com/2014/01/21/what-hard-drive-should-i-buy/ If you are willing to pay for reliability Hitachi seems like the way to go. If they fail they do it early in their life span, which seems like a huge bonus to me. Replacing data that is only a few months old is way easier than replacing it a few years down the line.
I had the first Barracuda. THE ONE. 9600 RPM, SCSI-1, 1GB, $2k in 1992. Fucker sounded like a lear jet powering up. It was without a doubt the hottest shit drive you could buy for any price. That drive pushed me through a good 15 years of Seagate nostalgia. I don't think I wised up until a couple years ago.
Probably 7200 (first to go that fast, I believe). There was a 5.25 drive made by the Seagate division in Minneapolis that develeped a drive called the Elite that spun at 10,000 - that sucker got really hot, but it was a nice piece of work. I worked for Seagate from '87 to '98, mostly on the Wren series (developed in OKC). But it was a division of Control Data when I started, Seagate bought us later.
Seagate had a crap rep when they first started, because all they made at first were stepper-motor HDD's. They bought Control Data's disc division (branded as Imprimis), to up their game (voice-coil actuators replacing stepper motors), and they phased out the steppers quick. That integration of Control Data was the beginning of Seagate's first swing of good reputation. edit - I was wrong about the Elite - it was earlier, and the first to spin faster than 3600, it spun at 5400. 3600 was the standard for a long time before that. I think it may have been a later 'Cuda that spun at 10K.
I haven't - in part because I'm not doing that much deep storage. It'll get there, I'm sure. That's more of a "planned backup" solution as opposed to an "incremental backup" solution - in an ideal universe I'd be doing both but I'm still busily feeling gobsmacked by dropoing two large on a bunch of spinning magnetism.