NVMe vs : What's the difference?

My M2_1 slot does not seem to be detecting my NVME SSDs, they work fine in the M2_2 slot Does anyone know why? I want to use both M2

2 NVMe slots These are the very fast interfaces for your SSDs The 'x16' on the expansion slots is the number of PCIe channels on

m2 slot mainboard 11Network Upgrade Slot Synology SNV3400 series NVMe SSD drives can be installed through the built-in slots to enable SSD caching or create SSD storage

m24 slot wallet Supports 7th 6th Generation Intel® Core™ Processors; Dual Channel DDR4, 2 DIMMs; Ultra-Fast with PCIe Gen2 x2 & SATA mode support; 8-channel HD Audio