Adding 3rd & 4th WD Red drives to my EX4 always fails

Adding 3rd & 4th WD Red drives to my EX4 always fails. Upon restart after insertion the lights above all the drives flash blue for a short while and then the original two drives stay blue but the two new drives become red and stay locked on red continuously. If I leave them in for long they get very hot.
I have a total of 6 WD 3TB Red drives at this point. My intent was to have the EX4 setup with two Raid 1s, and have a spare readily available. In addition I put one in my workstation. The original 2 in the EX4 are still solid, and the one in my workstation appears to be healthy (But it gets very little activity). However, I can’t get any combination of the other three to work in slots 3 & 4 of the EX4. Could all three be bad?

Because you are using an EX4 you may want to post to the dedicated EX4 subforum where those more familiar with the EX4 may help out. This suboforum (My Cloud) is primarily for the single bay My Cloud units.

This thread has been moved to the correct location.