Single Drive or RAID
The next key fork in the decision road is whether to choose a single drive or multi-drive RAID product. The RAID vs. No-RAID decision is not about data security! It's primarily about how much capacity you want in a single physical box.
You should never trust your precious data to a single device, no matter what RAID level (0, 1, 5, 6, 10, etc.).
RAID is not backup!
RAID was created to enable the construction of large storage arrays out of relatively inexpensive hard drives, using redundancy to compensate for the relatively short life of hard drives. RAID 5 and 10 arrays should be able to withstand the loss of a single drive without losing your data and RAID 6 should be able to lose two drives. But no RAID array will guarantee against data loss if the power supply or controller board fails, or the unit is physically damaged or stolen.
My strong advice it to not use RAID if your storage needs are below 3 TB; stay with single-drive products. If you need more storage, you can always add another NAS and probably still come out ahead for cost.
It might seem strange seeing this topic so far into the article, since some NAS buyers place performance above almost all other criteria. But many NAS shoppers agonize over the choice, when, in fact, other factors can make it moot.
The key performance criteria for NASes are read and write throughput. Throughput is affected by three things: NAS performance; Client performance; and Network speed. If the client or network connection is slower than the NAS, then you won't be able to get the full benefit from a very high-performance NAS.
NAS Performance is primarily determined by processor platform, then internal OS and filesystem used. What generally doesn't matter is the performance of the hard drive used. Hard drive access time, seek time, etc. are generally masked by the overhead of moving data across a network.
NAS RAM size helps only if the file size being read or written is smaller than the RAM size. So if you copy a lot of 1 GB ripped DVD VOB files, then you'll want a NAS with at least that much RAM. But generally, unless you are frequently writing and reading a lot of small files, RAM size won't matter much.
Client Performance is influenced by the same factors that affect NAS performance, i.e. processor, RAM and OS. I've found that a simple switch from Windows XP to Vista SP1 (or Windows 7) can make a huge difference in performance if you have a high performance NAS. How To Build a Really Fast NAS - Part 6: The Vista (SP1) Difference has the details.
The client also must have a Gigabit Ethernet adapter (on-board or separate card) that uses the PCIe bus. PCI-based adapters will limit performance to around 500 Mbps (~63 MB/s) due to overhead factors.
Network speed has to be fast enough to not be a bottleneck. For most current-generation NASes, that means a Gigabit Ethernet connection. This is because even inexpensive single-drive NASes can exceed the 12.5 MB/s limit imposed by using a 100 Mbps Ethernet connection.
If you're using a wireless connection, even if it's draft 802.11n, you don't even need to think about NAS performance; the wireless connection will be the limiting factor. This is because you're doing well if you get about 60 Mbps (Megabits/second) from a draft 11n connection, which is 7.5 MB/s (Megabytes/second).
As with routers, buying something faster than your network connection (or client) can support doesn't buy you anything unless you intend to remove those limiting factors. If you're looking at NASes capable of 50 MB/s or higher speeds, you'll really need to ensure that you client is up to snuff. This means dual core CPU, lots of RAM, OS that can use large block sizes for network transfers, PCIe Gigabit Ethernet and fast storage (SSD or RAID 0 hard drives array).
You're now at the point where the easy desicions have been made and where most people get brain freeze trying to pick a NAS. The main hangup is usually described by newbies looking for buying advice as "not wanting to pay for features that I won't use/ don't need". This is usually followed by a laundry list of features that they would like to have.
This need vs. want vs. cost question is one that only you can answer. But it helps to understand the basic platform types that NAS makers are pumping out.
The Econobox is a single-drive NAS that has all the What They All Do features and usually adds some NAS backup and Media serving features. They typically have Marvell processors.
The Compromise is somewhere in between the Econobox and Classic RAID5 and probably the biggest brain-freezer. It can be a single drive, but is usually a two or three drive BYOD with a smorgasbord of features. Processors can be from either Marvell or Intel Atom.
The Classic RAID5 is a four-drive box aimed primarily at small-business users and prosumers that are either convinced that RAID5 provides higher data security or prefer a lot of storage in a single package. It's usually closer to the Kitchen Sink in price and features, differing mostly in the number of drives and less powerful CPU. Processors are usually Intel Atom.
Kitchen Sink NASes are five-drive (and higher) RAID products that make your eyes glaze over while reading their extremely long feature lists, multi-core Intel processors and multi-Gigabyte RAM size They usually also cause sticker shock when you see their price tags.
The Server Killer is a variation of the Kitchen Sink. This is a very attractive target for NAS makers because the boxes they are aiming to displace are Windows Server based Dell servers or high-end NetAPP storage systems, which cost two to three times as much. The changes from Kitchen Sink models usually include rackmount form factor, redundant power supplies, higher end processors, more RAM, more drive bays and ability to support expansion cabinets.