I've read this entire thread because I have a similar issue. I have a mid-2010 MacPro (5,1) with a Samsung 500 GB SSD dedicated to system and applications, and 3 4TB HSGT drives. Everything mounts and works well with a cold start. Upon restart, as reported by others, one of the HSGT drives does not mount. The other two do. So far as I can tell, they are the same model (although the problem disk model number, ending in 614, is different from the other two):
HSGT HDN726040ALE614 (Device name in Disk Utility: "disk 4")
HGST HDN724040ALE640 ("disk 2")
HGST HDN724040ALE640 ("disk 1")
Upon restart, it is the first in this list that does not mount. This was purchased on 9/8/2017 (still under warranty). The other two were purchased in 2015 and 2016.
I reported this to OWC on Oct. 14 of this year (I had been living with this issue for quite some time at this point) to see if they had any comment. After some back-and-forth clarification (the OWC tech initially did not understand what I was saying) I got this response:
" I'm unaware of the issue with these drives that you are describing at this time, so I am glad you were able to point me to that.
With that being said, the first two drives are no longer under warranty, and while the third drive is under warranty, it's a drive that we longer carry. In fact, we don't carry many of the HGST drives at all anymore at this time.
I would point out the following as a solution for replacing the drives: https://eshop.macsales.com/item/Toshiba/MG04ACA400E/
This drive also carries a 5 year warranty with it, which is a great choice in cases like this. I'm unaware of this specific drive having the issue that you are describing at this time."
It is somewhat curious that they continue to claim unawareness of this issue. In any case, I will eventually try the recommended Toshiba drive and see what happens.