6TB drives won't mount after restart in Mac Pro 2012

The problem manifests itself when a 6TB HGST drive does NOT mount on the desktop nor appears in the Disk Utility after a reboot. The ONLY way I have figured out how to work around this problem is to shut down the computer and the open the case and slightly pull out one 6TB hard drive (i.e. enough to release it from the SATA connector on the logic board) and then push it back in. At that point, if I boot the computer any and all 6TB HGST drives installed in the computer will appear on the desktop until I restart the next time. I don't have to remove each and every drive to make them all appear, just one.

When I first experienced this issue, I assumed it was a hardware problem with that one computer, but I have now experienced it again with a different computer at the different company and with a 6TB HGST drive from a different production batch. Now that I have two data points, I want to test this configuration on other computers to see if I can figure out if it is more widespread than the existing two machines.

The similarities between the two machines are more than the differences. They are both Mac Pro 2010 5,1 models at their core, but they are also both the 2012 variant that was a slight upgrade from the 2010 model. They both have the shipping EFI firmware (which is the latest for that model). They both have 20GB of RAM and they both run Mac OS X 10.8.5. One machine is used for graphic design and the other is a server. They also both have more than one drive slot being used. The one machine that is the server have four drives. Two are 2GB drives used as the boot partitions. The last two are 6TB data drives. The other machine that is used for design has two 2TB drives for booting and backup and one 6TB drive for Time Machine (the fourth slot is empty). In both cases the 2TB drives are Toshiba drives (using the HGST derived manufacture process). Also in both cases the two 2TB drives are in slot 1 and 2 and the 6TB drive(s) are in 3 or 3 and 4. They both have ATI Radeon 5770 cards. Both are plugged into Ethernet networks and do not have wireless turned on.

There are some differences too. The server has a number of external RAID boxes connected via an ESATA card. The other computer has nothing in the PCI-E slots. The server has an after market replacement DVD and the graphics machine has its stock DVD drive. The server machine runs OS X Server.app.

I have a number of resources at my disposal so I plan to do some further testing. That said, I don't have everything to be really thorough and would love some help from others whom are either experiencing this or just want to help. Here is what I don't know and would like to test.

I have in my world Mac Pros that I can test at different customer locations. I can test a 1,1 easily, and I can also test a 3,1 and 4,1. I have those all available in my direct control right now. What I don't have is a 5,1 2010 model, but I can borrow one to test. I can also procure another 6TB HGST drive. What I don't have is a WD 6TB or other brand. I would love to know if this is specific to HGST or if it is a general problem for 6TB drives, Also, for that matter, I would love to see if this happens with 5TB drives. Has anyone else installed a HGST or WD 6TB and had mounting problems?

It would also be great to know if this issue is independent of OS. I plan to test 10.6, 10.8, and 10.10. Of course on the 1,1 Mac Pro I can't test past 10.7. I also would like to know if it matters that a 2TB drive is present and even if that drive is a Toshiba or other brand and even the quantity of drives and their slot position. I hazily remember trying to move drives around in slots when I first encountered this problem. I also wonder if different models of the 6TB drive may behave differently (i.e. NAS versus Enterprise).

I plan to post my results when I am done with the testing.