At the very end of thread that I linked, someone mentions using Tinkertool System to automatically mount the drives before login, and then using Cocktail to delay startup by 20 to 30 seconds. After discussions with Apple and HGST, no one expects any updates that would fix this issue. so, on a reboot, they are too slow to mount. The basic gist of that thread is that the SSD boots so quickly, and the 6TB HGST HDDs require a little more time to spin up. Turns out, I didn't do my research very well, and this has been documented before: Cold start boot - everything is fine again. Rebooted the computer - the 6TB HGST HDDs wouldn't mount. Booted the computer, everything worked great. I installed an SSD for the boot drive, and a couple 6TB HGST for data and backup. I'm doing some updates/upgrades to my wife's mid-2010 Mac Pro (5,1).
