Originally published 2 Sept 2009. Updated 20 August 2011
If you don't want (or care) to get into the details behind what makes a NAS tick and just want a short list of candidates to consider, then this article is for you. If you want a more complete guide to choosing a NAS, then read How To Choose the Right NAS for You. (That one hasn't been updated yet, though.)
What They All Do
Let's first list the things that all NASes support:
File Sharing - All NASes provide the basic function of networked storage—just like your computer does when you enable file sharing. But NASes have their own little computers in them and so use much less power, take up much less space and generate much less heat.
Client Backup - If you can read and write to a NAS, then a backup program can too. Some NASes come bundled with one or more licenses for a backup utility—usually for Windows machines. But if the NAS you're considering doesn't come bundled with a backup client, there are plenty to choose from.
Note that some bundled clients aren't very flexible or careful about how they load down your system when they run. So you might end up having to buy a backup program anyway. So don't let the presence or absence of a backup client affect your buy decision.
Ethernet Connection - All NASes also connect to a LAN via Ethernet. Gigabit Ethernet (10/100/1000) is now the rule vs. the exception. But not all NASes with Gigabit LANs support jumbo frames. Lack of jumbo frame support isn't really a biggie. My current NAS test setup that uses an Intel Core 2 Duo machine with RAID 0 array and PCIe Gigabit Ethernet frequently shows little performance gain using 4K jumbo frames and many times actually shows decreased performance!
SMB / CIFS support - The lingua franca of networked filesystems is SMB / CIFS, which is supported by most modern OSes thanks to the work of the open source Samba project. This means that a NAS will work with Windows, Mac OS X, Linux, etc.
The other main network file systems are NFS and AFP, which not all NASes support. Unless you're a 'nix user, you don't need to worry about NFS. You might want AFP support if your Macs are still using OS 9 or earlier.
Note that the internal drive format used does not matter! Clients don't know anything about how a NAS internally stores its data. The only thing that matters to the client is the network file system / protocol. And here, SMB/CIFS is king.
Web browser-based administration - As with most consumer networking products, NASes are administrated through browser-based interfaces. The main gotcha is that products tend to come with Windows-only utilities that help you initially find the NAS and change its IP address to match your LAN.
So if you're not running Windows, you may need to probe around a bit to find your NAS' initial IP address. Fortunately, most NASes come set to automatically acquire their IP address settings via DHCP, which reduces the range of possible addresses that you'll need to check. In most cases, you can log into your LAN's router and find the NAS' IP address in the DHCP client list.
Expansion via USB 2.0 - All NASes have one or more USB 2.0 ports. Among other functions, these ports can be used to connect a USB hard drive, which can be used to back up the NAS internal drive or be shared on the network. Some NASes also throw in faster USB 3.0 or eSATA ports to support those faster drives.
Multiple User Accounts - With few exceptions, all NASes let you create multiple users and assign password protected (or not) storage space for them. You can also set up "Public" folders that all users can access without a password.
NOTE! Just because you can set up Users doesn't mean that you can set up Groups. If you need this feature, be sure to check the NAS specs and features carefully.
Diskful or B.Y.O.D.
A key decision that will narrow your selection field is whether to buy a NAS that comes with hard drive(s) installed (diskful) or without (Bring Your Own Drive).
Diskful NASes are generally cheaper than BYOD, especially single-drive models. The main downside is that when the internal hard drive dies, replacement is usually a factory-only option. One year warranties might tip your decision toward BYOD models. But the three year warranties like those on multi-drive Buffalo and NETGEAR ReadyNAS products, might make it worth choosing a diskful model.
Bring Your Own Drive (BYOD) NAS' key advantage over diskful models is easy drive replacement, because they must have the ability to format and install whatever files are needed on any raw drive. Many multi-drive RAID NASes also let you start with a single drive and then add more drives as your storage needs grow.
But, again, you need to look carefully at product specs. If you are seriously considering the start-small-and-grow approach, I recommend you download the user manual of any prospective buy and read through the details of its RAID migration and expansion features. RAID migration isn't usually supported among all volume configurations. RAID expansion also is very time consuming and requires a volume resync after each drive is replaced. A four-drive RAID 5 expansion can easily take over a day using 1 TB drives. And unless you're dealing with a RAID 6 volume, every second that an array is resyncing leaves you open to losing all your data.
Finally, you can't necessarily throw those old drives that you have sitting on a shelf into your new NAS. For best performance and reliability, you should stick to drives chosen from a product's supported drive list. See Is That an Approved Drive In Your NAS? Or Are You Happy To Risk Your Data?