My server has two NVMe flash SSD drives. He had no problem using them with Ubuntu 16.04, but after reinstalling the server with Ubuntu 18.04, it only detects an NVMe drive. I changed the location of the slots of those two NVMe units, but the problem remained.
If I take out any of them, everything is fine. When I reinstalled the server with Ubuntu 16.04, it detected the two NVMe drives very well.
I wonder what happens to Ubuntu 16.04? I can not find any document that implies such an error.