"Absolutely not, we use software RAID controllers to constantly synchronise and monitor the array when RAID 5 is enabled.
"When configuring RAID 5, either on a server or a dedicated NAS product there is a lengthy synchronisation check that takes place to make sure everything is working correctly, you can even schedule this check to happen on our product every week if you so wish."
What does that actually mean? That they don't use ECC, but that you can check for inconsistent checksums a week after the corruption is irreversible? That's worthy of "Yes, minister"!
Or perhaps what they mean is, they use a hardware RAID controller, so that a RAM error in the memory attached to the network-facing CPU can't affect RAID-5 XOR calculations, but can only corrupt the data sent to and fro on the network without any sign of trouble. That's supposed to be better?
My question stands. Do they use ECC RAM -- for the network-facing CPU and even more critically, for the RAM built in to the RAID controller. No ECC, no warning that what was written into the RAM is not what came out, so one can suffer creeping corruption without any explicit hardware errors.
My own opinion of hardware RAID controllers is very jaundiced. I've had nothing but grief over the years from various Adaptec and 3Ware 4 or 8 disk offerings. Of course, past experience may not generalize to current products. Another nasty FAIL with such beasts -- if (when!) the controller dies, you may find that it is impossible to reconstruct the array by connecting the disks to a new controller (new model because the one you have is obsolete, or even same obsolete model with a new firmware revision). In contrast, with up-to-date Linux software RAID, you just attach the disks to a new motherboard and everything auto-assembles and works. Combine this with the advanced mdraid features like resizing and reshaping, and I'll go for Linux-based software RAID every time.
And don't get me started on fake RAID controllers - pathetic one-function chips that calculate XORs at a small fraction of the speed of the cheapest Athlon you can buy! They might have been a good idea when state-of-the-art was a 100MHz i486. They're snake oil today.
(Enterprise-class hardware RAID and NAS is a different kettle of fish - I'm flaming about NAS boxes that cost a couple of grand tops here.)