Hi, just tuning into this thread. I've had very similar problems with several large HDDs, which include:
6TB WD Black
8TB HGST He8
8TB WD Red (White OEM label)
When mounted in 3G internal bays (Mac Pro 2012 tower), all mount on cold reboot, but not on warm restart. Does not matter which OS (10.10.5 vs 10.13.3) or where the system resides (SSD on PCIe interface, SSD on drive sled, or one of the HDDs).
6TB WD Black
8TB HGST He8
8TB WD Red (White OEM label)
When mounted in 3G internal bays (Mac Pro 2012 tower), all mount on cold reboot, but not on warm restart. Does not matter which OS (10.10.5 vs 10.13.3) or where the system resides (SSD on PCIe interface, SSD on drive sled, or one of the HDDs).