Large hard drives won't mount after restart in Mac Pro 2012

This post closely follows a previous thread: 6TB drives won't mount after restart in Mac Pro 2012

I have had a very similar problem with a Mac Pro 2012, running either 10.10.5 or 10.13.3 off a SSD. Upon restart or Option-restart, the system will not mount the following disks:

6TB WD Black WD6002FZWZ

8TB HGST He8 HUH728080ALE600

8TB WD Red (white labeled OEM version WD80EMAZ)

Other details:

- On cold boot or option-boot, all of these drives will mount, and all the drives work great.

- Two different systems are affected exactly the same way: Yosemite HFS on SATA 3 sled or High Sierra APFS on PCI card.

- None of the drives are formatted involving a Logical Volume Group (this was a red herring in the last thread).

- One of the effected drives (WD Red) has a 2TB partition with Yosemite on it. Upon option-restart, this partition shows up on the option screen and is selectable. If selected, the system gets part way through startup, then a grey circle with line through it appears. This would seem to imply the problem is not a time-out waiting for the drives to spin up as has been suggested in the previous thread. Rather something is causing the drives to shut down or unmount part way through startup (including the startup partition).

- 2 of the 3 drives are SATA 3.3 compliant and have a remote power-off on pin 3. But in all cases pin 3 is manually taped off (and the problem does not occur on cold restart), so I am sure it is not a PWDIS issue.