Yeah it's obvious in retrospect but a lot of people don't get their head around the fact that most NAS boxes (proper ones) have internal processing and it's likely to be Linux based and will almost never use a Microsoft filing system. They should never rely on your PC for processing.
For future reference aside from the things that worked in this instance if you suspect a dud drive and linux tools do not help then remove it from the NAS, hook it up to your Windows PC (some kind of USB to SATA device would be handy) and use
SeaTools to give it a good test. It doesn't apply so much to RAID 1 but if you move on to levels 5 or 6 eventually proving or disproving that a drive is duff can prove invaluable (I've just spent 3 weeks doing this to my iTunes NAS).
Good luck recovering the data, no reason it won't go well at this stage.