PR4100 - RAID or JBOD for Plex

We purchased a PR4100 to use with Plex and put 2 WD Red 6TB drives (left 2 slots empty) set as JBOD. I am about finished transferring our fairly extensive DVD and BR shows on the NAS. This process took significantly longer than I expected. Now I’m concerned about having a back-up. I originally expected that the discs would be my back-up, but the time and effort of re-populating the NAS, in case of disc failure, is daunting. Also, the data on the NAS is nowhere near the 12TB capacity that we currently have (only using about 2TB).

My first question is: when 2 drives are in the PR4100 as JBOD, how is the storing of the data done? Just on the first drive? or can each file span both drives?

My second question is: if I wanted to back up the data would I be better off purchasing 2 more drives and creating a RAID? Or maybe purchasing another drive, and doing a simple copy.

Please excuse my ignorance. My brother used to handle all our tech issues, and he is no longer with us. So I’m having to learn all this myself.