External (Thunderbolt 2) drive keeps changing to read-only randomly
I assume you've already done this, but I would check my connectors.  Additionally, the one drive may not be getting as much power and/or can be beginning to fail.  If you have a way to test it externally on another device, I think we would learn a lot more about the cause.
I hope this provides some helpful ideas/thoughts.  Keep us posted!