Just for fun, nothing can be 100% reliable. Look at the budget NASA has. A Hard drive has a MTBF rate. It will die.
100%, does that cover fire, flood, space debris?
Just for conversation. 2 drives provides the same “concept” as 10 drives. You are protected if a disc dies. With a raid 5 three drives, you have the same protection if one disc dies.
The next concept is a self healing array. As in if a drive gets a bad spot, it gets the good data from the other drive(s) and relocates it. This takes huge overhead as the drives are no longer “Mirrored” and it requires a databse to keep up with all the relocations. Where do you put this database? What happens to this database when the power is removed from it?
And just for fun, if you say backups. How do you get the data from the PC to the NAS? As in the only way to get a “perfect” backup is to take the PC offline and do a cold image. No way a vendor can provide VSS writers for every application out there.
The best concept is mutiple backup targets with one possibly being offsite and then you have to do regular test restores.