consumer grade optical disc. But not as individual drives. At the moment, a multi-drive, RAID1 optioned, Network Attached Storage (NAS) is about as stable as it gets. But they are not inexpensive. Basically, there are two hard disc drives in the NAS which are identical. The chances that both will fail at the same time are extremely slim. However, when one dies, replace it (hot swap) and the remaining working unit copies its contents over automatically. If you really want to get in to it, you can take one of the working drives out and send it somewhere for storage. This way, there is one "set" somewhere else (not in the drive).
Since regular optical discs (DVDs) don't hold much (4.7 gig and 8.5 gig for single layer and double layer, respectively) and since their adhesives can delaminate it would be prudent to look into Blue Ray blanks... 25 and 50 gig sizes are available. But you still run into the same file fit issue. If you can afford a NAS, then a 4-bay where each drive can run a 4 terabyte drive (remember, the drives work in pairs), then you can start with a 4-terabyte pair and add another pair later.
Seagate, Buffalo, Iomega, Netgear... they all make them... stick it in a closet and add it to the network. They work great. You can put all your other stuff on there, too... photos, music, movies... secure it, make accessible remotely... lots of other features and flexibility... but at a price...