What other drives do you have in the system? Please specify which drive has OC and where the OS is. (Internal bay, USB, PCIe)
Thank you for your reply, rroumen (and VaporShark and startergo),
Yesterday, I had 4 drives in the Mac Pro 5.1, but this morning I pulled two HDDs out because I thought that might be the problem, but it still doesn't work after I do the SMC reset. The physical drives now in it are:
Bay 1: 2TB SATA hdd containing a working installation of the latest Mojave (and Catalina installer). I believe this HDD was formatted with the APFS scheme a week or so ago when I installed Mojave on it (it was a new drive) along with a metal-capable GPU (Sapphire Pulse Radeon RX580)...yes, I'm coming to the party late!
Bay 2: 120GB Samsung SSD, freshly erased and partitioned (one total partition) using Disk Utility (GUI version included with Mojave). I successfully mounted the EFI volume on that SSD (disk1s1 in my case), and copied into that volume the EFI directory from the latest OC download, and edited/saved config.plist and put it into EFI/EFI/OC ... and did all the steps on the first page of this thread, seemingly successfully, but after I unplug the computer to reset the SMC and restart, I get the chime, and the fans run ... but nothing beyond that. (How long does it typically take to boot at this stage? I have 32GB of error-correcting RAM, if that causes it to take longer to boot.)
One thought: I have a USB3 card in one of the slots that I added a year or two ago...could that be causing an issue?
Alternately, is there any way I could use my Macbook Air (13 inch, early 2015), hooked up to my OWC "Drive Dock" (I added the USB3 card to interface with that), to install Catalina on my SSD, then pull that SSD out of the Drive Dock and install it into my Mac Pro...possibly with some help from OC?
Thanks in advance for any help, and I apologize if these are n00b questions/problems.
ETA: One other thought: Could it be that my firmware never successfully updated? A week or so ago, during my upgrade from Sierra through HS to Mojave, I had a devil of a time trying to get through the "firmware update" process. First I couldn't get the FW update to work by holding down the power button, then it FINALLY started the FW update process but seemed to freeze up 90% of the way through – but it allowed me to proceed with the install of HS or Mojave (I forget which), which led me to believe the firmware had successfully updated – but then somewhere along the line it told me I had to update firmware AGAIN...sorry for the fuzzy details, but it's all kind of a blur since I've made so many changes, and had so much difficulty doing so, over the past few weeks. I held off on putting in the new GPU until I was ready to upgrade to Mojave, but upgrading to Mojave required me to first upgrade to HS, which in turn required the firmware update...it seems like Apple forces you to go through all these doors, each of which closes and locks behind you as soon as you go through...