I see that people are still reading this review after a couple of years, and I appreciate all of the positive feedback, so only fair to keep it up to date.
I stick with most of what I say below, but will be switching to a Home Server running Windows Home Server 2011. The NAS is fine as a simple file server or backup device, but the technology is old and the laborious procedure for updating the Squeezebox software (main reason I originally bought the DUO) has become too much of a niggle, and my network has expanded. So it is time to move on, and I can no longer really recommend this NAS with complete conviction.
Original review:
Very important: Start with 1 disk only in the ReadyNas, and if you have 2 disks of different sizes that you intend to use, make sure you start with the SMALLEST one first!
Also: It really is very important to read the instructions!
This is an excellent little NAS device for the price, but does require a bit of prior knowledge to set up or research if you are not overly familiar with networking, and you should definitely take the time to read the manual on the installation disc. First you should be aware of the limitations. For this price you are not buying a high performance device, but if you are willing to spend a bit more you can upgrade the Duo to get a lot more from it. I'll come back to that later. You must also read the documentation to see what hard drives and what combination of drives are compatible. I've seen a couple of reviews here on Amazon with people complaining that this or that doesn't work. That's simply not true but you do need to perform some basic checks before you start spending money. I purchased the RND2000 which ships with no drives, but then purchased a couple of 500GB Seagate drives (serial ATA running at 7200 RPM). You could put an old drive in it but large new discs are fairly inexpensive these days, and they are making them much more energy efficient now too. You will have to completely format any old drive before beginning anyway as the file management software uses a proprietary Linux operating system.
Do visit the ReadyNAS website as it contains a lot of useful information. I bought this to have a backup solution, as well as the ability to share information between computers on my home network. I have not yet tried the internet file sharing facilities so cannot comment on those. I also wanted a music library for my Squeezebox that is accessible while my pc is switched off.
For backup you should be sure to install the NTI shadow software, which is excellent by the way, from the disc before you attempt to back up your files. You should also read the manual to learn how to set up folders in the ReadyNAS before you back anything up. NTI shadow is then a doddle to use, and once the initial backup is done it will do incremental backups so that only the files that are changed or added will be backed up. For the initial backup you need to give yourself some time to get it all done, and if you have a large amount of data this means some hours (you can of course leave it to get on with things while you wash your hair or watch the football).
The next thing is the Squeezebox set-up. I can't comment on the other add-ons such as iTunes and Bit Torrent as I don't use them, although I've read in many reviews that the Bit Torrent client works fine. First thing is that the version of Squeezecenter that comes embedded in the ReadyNAS Duo is a light version, and if you have a Squeezebox then almost inevitably you have already upgraded to the new and renamed Squeezebox Server. What this means is that if you backup your music files to the Duo and then access it from the Squeezebox, the firmware on the Squeezebox will update to the older version, and if you then switch back to the library on your PC it will have to undergo another update to the new firmware. This will happen each time you switch libraries so obviously it makes sense to upgrade the firmware on the Duo to be the same as on your PC. This is quite easy to do but you will have to follow the instructions carefully. There is a link in the Duo control panel under "Streaming Services" next to the Squeezebox settings which when clicked will take you through a step-by-step procedure to successfully upgrade. Once this is done the interface with the Squeezebox is seamless and it's wonderful to have your music available on demand. There is a downside though, which is that every time there is a new release of the Squeezebox Server and you decide to upgrade the version on your PC, you will have to completely remove the server software on the ReadyNAS and re-install the newer version after following a fairly lengthy procedure if you want both versions to be syncronised. It is also worth noting that the version releases for your PC and the ReadyNAS may not be at the same time so if you do intend to upgrade one, then it's wise to check if the other is available too.
Now a few words on beefing up the performance. You should be aware that the Duo ships with a category 5 LAN cable which is only capable of transferring data at a rate of 100Mbps, but the device itself is capable of 1Gbps. This means that if your router is capable of 1Gbps you should also purchase a Cat 5e or Cat 6 LAN cable to get the most out of it. You should also be aware that if you are accessing the Duo over a wireless connection the speed of data transfer will only be as high as your router and wireless card are capable of. It will be much faster if your main PC is attached to the router via a LAN cable (which should also be cat 5e or 6 to maximise speed), but if you go wireless then a wireless N setup will be faster than wireless G. The next thing you can do is replace the 256MB RAM with up to 1GB. You do need to take care that it is compatible. There is a compatibility list on the ReadyNAS webite (not to be confused with the Netgear website), but the module I used was purchased from Crucial Memory, the part number is CT12864X335 and it cost about £40. If you only wish to upgrade to 512MB then you will have to use the compatibility list to see what is best.
So in summary, this is a lovely little device that is easily upgradeable, but it's not plug and play and does require a small amount of confidence to set up, although very easy to do if you are willing to read the documentation properly. If you are also willing to spend a bit more you can turn the ReadyNAS Duo into quite an impressive NAS device for a reasonable cost. Did I also mention that it is tiny? About the size of two Wilbur Smith novels put together! It runs very quietly after a noisy start, and the power saving options work with no trouble. I think it is excellent but gave it 4 stars as you need to spend more to get the best from it and is not as simple to set up as some may like.
It is also a bit of a pain to keep the Squeezebox Server versions updated and in line with the version on your PC, so if this is one of the main reasons for buying the ReadyNAS then there may be alternatives out there that are a bit less work.