I think that's precisely what I am missing. I missed the NAS bandwagon so I am clueless as to what I'd be missing. What would I need that I don't already get backing up to a simple 2-HDD RAID1 connected directly to the Mini, since it's already doing all the fancy media streaming, web serving, etc.? Or a 3- or 4-HDD enclosure to cover the scenario you mention, 2xHDD in RAID1 and a third for backup. I am sure I am missing something major. I also can't see how the NAS is cheaper.
I would imagine whatever it is would be partitioned to set aside, say, 2TB as Time Machine for the Mini (256GB SSD), MacBook AIR (256GB SSD), and MacBook Pro (512GB SSD), then leave the rest for the "other": the photo/video libraries, business/legal/tax, etc.
It's pretty funny, I checked the DSLR photo/video libraries and they only consume 550GB as of today. But going forward I want to be much freer to capture the (4) kids, create media for our business, etc. as well as include photo/video coming from other devices such as our phones so all can easily be used in the editing process.
All together I would think 4TB should be sufficient. For offsite something like a HGST Deskstar should be OK, no?
So using Kevin's example, if I had a 2xHDD RAID1 and a third drive to back that up for offsite, assuming I have sufficient space, I'd be pretty close, no?
Then the "fast access" system could be a Thunderbay, LaCie 5big, Pegasus, or whatever. Yes, for that purpose an 8-bay in RAID6/10 would be pretty awesome and great performance/capacity/protection balance, but that's really getting up there in cost.