Re: Let's run the numbers...
If using a NAS then you need to consider what happens when a HDD fails, and often the raid rebuild will cause others to croak (or at least reveal sector errors). So you should:
1) Use double parity if at all possible (i.e. RAID-6 or similar like RAID-Z2).
2) Perform regular scrubs (i.e. weekly check where RAID system reads all disks and repairs any sector errors).
3) Use ZFS please, as it has much better error checking and correction, and it will tell you which files are trashed (which a lot of file system's don't).
4) Use a server with ECC memory (OK, getting expensive I know...)
I'm not kidding, see the following list of papers: