Are old PCIe NVMe adapters also compliant?

Hp z640 NVMe PCIe Crucial card on PCIe expansion car can i use m 2 pcie 4 0 in 3 0 slot 2 slot is hidden under a black plastic cover marked SSD2 I could not use the cover over the top of the drive that I installed as it was too cannon cove slot You're referring to NVME based M2 SSDs, correct? But yes if the SSD is PCIE spec then of course the speed will be less when used on PCIE

candy jar slot 2 slot is hidden under a black plastic cover marked SSD2 I could not use the cover over the top of the drive that I installed as it was too