New SSD install causes HDD to "vanish" from Finder

I may have exceeded the number of drives Sierra will support.

I have two Super Drives and 4 1TB HDD drives (one in each HDD drive bay) in a mid-2010 Mac Pro (Cheesegrater); these HDDs were recently replaced and function perfectly. Recently I installed a couple of new 1TB SSDs (using the PCIe slots). Installing the 1st SSD was uneventful. The 2nd SSD works fine, but installing it caused the HDD in the 4th drive bay to “vanish” from the system - if I remove an SSD the “missing” HDD reappears.

Also curiously enough, the 2nd SSD shows up on the desktop as an external drive, even though it’s installed into one of the internal PCIe slots.

Is there a hard limit to the number of drives Sierra (or the Mac itself) will support?