How is an Apple different from an Pig?
RAID-x and COW are completely different concepts. Copy-on-write is a technique to reduce disk storage and improve the efficiency of buffers. Currently it is particularly attractive for files providing the images to virtual machines and containers. If you have say a 200MB file system image, and you start 2 different virtual machines from it the straight forward way would be to make a copy of the 200MB for each and let them alter it however they wish. This will need 600MB (200 for the original and 200 each for the copies).With a COW based system then again you make a copy but this time with the cow-copy command. This just will need 200MB plus some bookkeeping overhead, until the VMs actually start writing.
COW can run fine on a single disk, although systems running lots of VMs will benefit from additional drives to allow multiple I/O operations at the same time. Wikipedia
RAID allows you to have multiple disks to improve one or more of size, speed and redundancy compared to a single drive. Wikipedia