I think I answered my own question, based on the service manual itself. Both slots support PCIe NVMe, whereas only the left slot also supports SATA AHCI.Since I already have one PCIe NVMe in the left slot, I am free to add another in the right slot.
According to the spec sheet for the Alienware m17 R2, the two M.2 slots have different interfaces. One is PCIe Gen3 0x4 NVMe (up to 32 Gbps) and the other is SATA AHCI (up to 6 Gbps). But I can't find any information indicating which is which. From what I've read, it seems that both slots should be able to support an M.2 Gen3 drive such as this one with similar performance. Is that correct? Can either slot support a Gen4 drive? If I can only use the slot with the PCIe interface with that drive, which slot is it? This is all very confusing to me, because I thought that M.2 was an interface that was distinct from either PCIe or SATA. If the slots are different, where is the documentation to tell me which is which??? Thanks for any help!
m17 R2, differences between M.2 slots?