NAS device claims drive in a RAID is degraded but S.M.A.R.T. says it is fine

S.M.A.R.T. can be used as an indicator that there are drive problems but can never be relied upon to indicate that a drive is good. When there is disagreement between multiple diagnostic systems always favour the one that shows the worst results.

As mentioned before, S.M.A.R.T. is great for alerting you when there are problems, but it definitely is not the catch-all solution for drive testing. I would remove the degraded drive and run tests against it to see what the issue is.

I recommend using SeaTools http://www.seagate.com/support/downloads/, there is a version you can run in Windows, or a bootable ISO if you want it to be OS independent.

The short test of SeaTools will do some basic checking, such as S.M.A.R.T. tests among others, but this will not guarantee to find errors on a drive, if they exsist. Running the long test will take a bit longer, but it will test each individual sector of the drive, and is MUCH better for finding out if a drive needs to be replaced or not. I've had many drives that have not had S.M.A.R.T errors and pass the short test, but failed on the long tests.