Can anyone suggest a way that might let me recover from this error?
Have you checked the drive and file system on the NAS? It could be hardware failure.
OP, you could check SMART status of your NAS HDD as starter point. It could be done by installing software (such as CrystalDiskInfo) either on your Windows box or NAS. And FWIW, it could be simple faulty cable which connect the NAS and Windows box.