Hard Drive Icons in Dock

I have a MacPro v3.1 running El Capitan, and four internal drives. The drives appear on the desktop, ok.

However, when trying to place these four icons in the Dock, only three make it! Trying to insert the fourth or missing drive icon within the dock only results in re-arranging the other three. If I have disks 1, 2 and 3 (in that order) in the dock and I try to place drive 4 the result is still three drives but now they are in this order 2,3,1.

What's wrong?