I wouldn't trust flash drives for a whole library of hard-to-replace music, they fail all the time and they're hard to recover once they do. Better to use a NAS, external hard drive, or cloud storage you pay for like S3.
It only protects you against disk failure, and it is mostly used to make sure the data is still available as you change disks. Not for long term data protection. Overall, a RAID NAS is probably not that safer than a standard external hard drive. RAID is mostly there to make sure that you can benefit from high performance, high capacity and networking without sacrificing reliability.
If you have two disks and availability is not a problem (you can afford not to listen to music as you restore from backup), you are better off with an external hard drive used only for backup/restore operation and kept somewhere safe when not in use.