I have a small (<500 GB) photo collection that I keep on a 2X3TB WD RAID1 and backup to the cloud. (My Lr catalog is on my Macbook, backed up separately). I now understand that there are severe downsides to depending on a RAID backup only ;e.g., a mistaken delete is instantly mirrored. My thought is to reformat my external drives from RAID to Drive A and Drive B and use 3rd party software to backup from my main collection (A) to the drive backup (B), all the time using the cloud for safety.
What is wrong with the setup plan? I am trying to avoid buying another 3TB drive for backup (Thunderbolts still aren't cheap), but maybe I am being penny wise and pound foolish.
I suggest you just buy a USB 3 drive and forget the thunderbolt system/connection (That is if your Mac is new enough to have USB 3 or USB C).
Thunderbolt is basically a DEAD technology. Being replaced with USB C
As for setting the WD external as to independent drives I thing that would be better. At least for more storage capacity.