I thought I'd throw in a pitch for building your own NAS using an old PC, and FreeNAS (or it's successor, NAS4Free). They are the only NAS software built on FreeBSD which allows you to use the ZFS filesystem (you can also use native FreeBSD to get ZFS, but you's better be a Unix command line wizard). ZFS is a very reliable file system, it maintain checksums on each bit. Most other filesystems just verify the data they are writing to the disk, but do not know if the data on the disk has become corrupted.
My NAS is an older Athlon II 3Ghz system with 8GB RAM, and using six 4GB drives. I boot NAS4Free from a Compact Flash card attached to a CF reader on an add-in PCIExpress SATA card, this leaves all six SATA ports on the board available for data drives. The drives are configured in two sets of three mirrored drives, all in one ZFS volume, so I have the capability of losing at least two drives without losing data. I use the most reliable drives I can find, at the moment according to Backblaze's data are Hitachi (HGST) drives. The NAS easily handles 100MB/sec transfers over Gigabit Ethernet, with the CPU utilization never exceeding 25%. The only problem with any roll your own NAS (and low end commercial NAS's as well) is they use non-ECC RAM, so you could potentially corrupt your data as it is cached in memory while copying it. This is pretty rare, but the bigger files get and the more data you have, the greater the risk of corruption. The only solution is to buy server grade hardware that will support ECC (Error Correcting Code) RAM, which can correct a single erroneous bit in a computer word with error correction circuitry. This is usually expensive stuff, but AsRock now makes some good high end workstation boards for about $200 that can use ECC memory, and ECC is a lot easier and cheaper to buy these days.
Having said all that, I work with computers for a living, most people will just buy an off the shelf two drive NAS and be done with it. Not a bad option at all, just make sure you set up the drives for mirroring so you will not lose data if a drive fails.