I replaced the broken unit with a WD 6TB since Amazon did not have a 8TB available AND I do not want to give up the benefit of having Amazon replacing failed units with no question asked, and quickly.

I'll think where to go from there. I may get a second WD 6TB and use this 8TB for something else, or just accept that 2TB are "wasted" in my mirror, until next upgrade. I mean, I always have something to do with HDDs...