Problems upgrading Mac Pro 5.1 with SSD system drive

Hello, I'm in the process of upgrading an older Mac Pro ("cheese grater") with Samsung QVO SSD drives. I've run into some weird and perplexing issues that I haven't seen addressed or even mentioned anywhere. I’ll try to keep my explanation brief, but this has been a long and complicated troubleshooting process, so bear with me. Skip to the end for a summary.

Here's the setup:

Mac Pro 5,1 Mid 2010

2 x 2.66 6 Core Xeon

24GB ECC RAM

ATI Radion HD 5770

Pro Tools HDX PCIe card

It had two internal SATA system drives in bays 1 & 2, one running 10.9.4, the other with 10.12.6.

I installed two Samsung SSD drives in OWC drive sleds in bays 3 & 4: a 1TB partitioned equally for the two OS versions, and a 2TB for storage. I used Carbon Copy Cloner to copy both original system drives over to the new SSD and tried to boot into them.

First off, I was able to boot into both of the old drives and the new 10.12 partition just fine. However the new SSD 10.9 just gave me the “universal no access symbol” after loading for a while on the apple screen. I was able to boot in safe mode but otherwise couldn’t get it to load normally. Also, when I booted into the old 10.9 neither of the new SSDs showed up anywhere, as if they weren’t even installed (they were available in boot select). This wasn’t an issue in either 10.12 partition. After some troubleshooting I discovered that if I unplugged all extra USB devices then the SSDs would show up again in the old 10.9, but I still couldn’t boot from the new 10.9. I tried several different combinations of USB devices and it didn’t seem to be any one in particular causing the problem.

Then I noticed that there are metal tabs on the motherboard next to each of the SATA ports. These tabs are depressed when a regular 3.5” drive is installed, but not with a 2.5” drive which is substantially thinner. I put some electrical tape on those tabs to hold them down and voila, the new drives were available in the old 10.9 with USB devices attached. This is obviously some kind of sensor but I’m not sure what it’s for. I’m guessing to monitor drive heat, or possibly confirm that a drive is inserted. There’s a SOT-23 device under the tab, but I can’t identify it. Either way, it seems to disable the port if there are too many USB devices. After some more troubleshooting I found that if I booted one of the working partitions and selected the SSD 10.9 as the startup drive in system preferences, then I could boot into it successfully.

At this point I thought I’d figured it all out, so I removed the old SATA drives and moved the new SSDs over to bays 1 & 2. This is where things got really confusing. Now I could boot into 10.9 fine, but I got the no access symbol when I booted 10.12. After more troubleshooting, I found that I can boot either OS only if I completely shut down first. If I restart then neither will load (this is true when restarting from one into the other). This is independent of USB devices.

I’ve done multiple SMC resets, and I applied the 10.13 firmware update, but still having issues.

So to summarize:

There’s a metal tab next to each internal SATA port that is depressed when a drive is inserted. Any idea what it’s for? I haven’t found it mentioned anywhere online. On this particular machine, if it’s not depressed then when I boot into 10.9 it disables the port if too many USB devices are attached.

I have 10.9.4 and 10.12.6 installed on separate partitions on one SSD drive. I can boot into either one if the computer is shut down first, but can’t if I restart. It just gives me the no access symbol after loading on the apple screen for a while. Is it a hardware/firmware issue? Having both OS versions on one drive?

Again, sorry for the long post. This was about a day and half of solid troubleshooting. I greatly appreciate any insight into these issues.