I had previously bought the 1TB Buffalo Linkstation ( a very decent NAS ) which I eventually filled up. Therefore, I decided to buy the 4TB duo which I paid a lot more for than, the price currently shown on Amazon, however I can't really complain about its performance.
Data transfer speeds ( backing up, copying etc) are higher than the 1TB version plus it also has the same useful bittorent and print server facilities.
It not the quietest nor the most solidly built NAS I've ever owned but after sticking 4 rubber pads to its base to absorb the vibration noises it's now less of a problem.
At its current price on Amazon, it's now even better value than ever for a high capacity NAS.
I've since bought a second 4TB duo due to the incredible feat of filling up the 1st one up. All my previous comments apply except the latest firmware updates have slightly improved on the 1st one's performance plus I paid £153 less.