Hello, I'm having some issues with booting my Mac Pro 5,1 from an Apple SSUBX SSD installed in a PCIe adapter. The SSD is Apple part # MZ-JPV2560/0A4 (655-1858H) and the PCIe adapter is this one: http://r.ebay.com/klbBjL. I have it installed in slot 4. When I boot after the chime is goes to the white/gray screen and hangs there for about 1 minute. Then it proceeds to boot as normal. When it's up and running it works fine and the speed test is good. But the hang up at the boot is annoying. I put it in slot 3 and when it's done hanging it just flashes the missing folder symbol. If I restart it holding the option key to select the boot drive that will come up and I can select it and it boots normally. I went into system preferences and made sure the drive is selected as the start up disk. But when I restart it does the same thing with the missing folder symbol. So that seems really strange. I put it back in slot 4 and it just has the hang up and boots normally again. I haven't tried putting it in slot 2. I've reset the PRAM, even 3 times in a row and it didn't change anything. But the computer wouldn't boot after I did the reset. It just had a black screen. I had to turn it off and unplug everything before it would boot again. Is that normal? Is there a way to tell if the SSD is good? I did a SMART test and that checks out without errors. I've also ran the Apple Hardware Test and it found no errors. Without purchasing another SSD, is there a way to tell if the adapter isn't good? I would prefer not to send the SSD back if it's good. --- Post Merged, May 14, 2017 --- I put it in slot 2 and it does not hang on boot. Is it normal for PCIe SSDs to act up in slots 3 and 4? --- Post Merged, May 14, 2017 --- So now I have a different problem. The speed test shows it's operating at half speed in slot 2.