Mac Pro 5,1 and SSUBX SSD from 2014 MBP

Any reason why a 2009 Mac Pro with firmware update to 5,1 will boot from a SSUBX from PCIe slot 1 or 2 but a 2010 Mac Pro 5,1 will not?

The 2010 Mac Pro has only successfully booted from slot 1 or 2, 2 times out of 20 or so attempts. Logic board issue? Firmware issue? Some sort of compatibility issue? I have no clue. Both Macs can boot when the SSD is in slot 3 (right above the graphics card) but for some reason the read/write speeds are slower than slots 1 and 2. Thanks for your help.

2009 Mac Pro

6 core 2.93 ghz

16gb 1066 MHz RAM

mac OS HS 10.13.4

2010 Mac Pro

12 core 3.46ghz

64gb 1333MHz RAM

macOS HS 10.13.4

SSD being used is from a 2014 MacBook Pro 256gb SSUBX.