I use 3-3TB WD Red drives in a homemade Windows Home Server 2011 (Intel MB, Intel I7, SSD boot drive). I run Stablebit Drivepool which allows one to pool the disks into a JBODs arrangement, but also to specify folders that should be duplicated. If a disk fails, you will lose anything not duplicated on that failed disk, but the duplicated folders would be available on the other disk. And there is no need to rebuild the array with the risks that carries or requiring using the identical RAID setup forever.
I am looking for a better ALMOST Archival solution, but have not found anything.
Late for this discussion but a few thoughts:
I'm on a new macpro using ssd for apps and time machine/CCC as backup for this.
Image archive Areca tb-2 with hardware raid allowing me to stripe/mirror all my photo work. Currently 16tb in raid 10. With WD blue enterprise 4tb drives eight total with a spare. Long time to build approx 24hrs, but one and done. Hot swappable with spare drive. Areca case avail thru B&H.
Carbon Copy Cloner B/U to Promise 4 bay case with 16tb mirrored to 8tb. Tb2 connected. Got the case when Apple had bare case for sale. Now you need to buy the case with inferior drives so so I would consider a second smaller Areca case for this purpose. These are not NAS. Also have used WD Thunderbolt Duo for backup swapping out the green drives for better performance.
Off site, I alternate bare 2tb blue WD drives (WD Se) in Thermaltake docking station.
For downloading cards I have started to use Lexar hub with their surprisingly affordable ssd module. Very time saving with today's large file sizes and multiple cards.