...OK. While rpm seems obvious, I am curious as to why the # of platters from the physics standpoint. I'd guess it is friction added by the extra platters, but I'm surprised by the impact this has!

Also, temp delta decreased to 5 degrees as the mirror rebuilt progressed, and stayed at 5 for hours yesterday until I went to sleep. I just woke up and NOW it is only 1 degree (72% completion).

HDD are in two adjacent trays, 8TV top, 6TB bottom. They are at 54 and 55 degrees respectively (Celsius).

Incidentally, I am thinking to add a fan right on the side of the trays.