Skylake PCIe lanes

Hi Hindesite,

I am sorry but my answer will not provide you an assurance of 100% (I am no expert in Alienware), but I can tell you what I think it should happen base on my knowledge and on comments identified in other forums.

First of all, the following comment should answer part of your question:

The alienware 15 R2, as well as the alienware 17 R3 which both have the same motherboards, have two m.2 slots.  One of the two slots supports both Pcie X4 and SATA III as the m.2 interface can support either sata or Pcie.

The second m.2 slot supports ONLY sata III and/or Pcie x2.  

To repeat, one of the slots supports 4 lanes of Pcie, the other supports only 2 lanes of Pcie.  Both slots support Sata III.

This means you can use the 850 EVO sata drive in either slot, or the 950 pro in either slot, however, in one slot the 950 pro will ONLY have 2 Pcie lanes available to it so it will not work at its top speed.  In the other slot it will have 4 Pcie lanes available to it and will work at its top performance.

Source

In regards of your second question:

Are there enough lanes to make this feasible, especially with other external ports in use (GA, DP/Thunderbolt)?

PCIe uses a point-to-point topology, so each lane expects one device on each end.

PCI is a bus, whereas PCI Express is a point-to-point connection, i.e., it connects only two devices; no other device can share this connection. Just to clarify, on a motherboard using standard PCI slots, all PCI devices are connected to the PCI bus and share the same data path, so a bottleneck (i.e., performance decrease because more than one device wants to transmit data at the same time) may occur. On a motherboard with PCI Express slots, each PCI Express slot is connected to the motherboard chipset using a dedicated lane, not sharing this lane (data path) with other PCI Express slots. Also, devices integrated on the motherboard, such as network, SATA, and USB controllers, are usually connected to the motherboard chipset using dedicated PCI Express connections.

Source

As a Summary:

A second M.2 960EVO shouldn't work at 100% of its capabilities because the second M.2 slot has 2 PCIe lanes instead of 4.

Other external ports in use should not affect or be affected by using your second M.2.

As commented, take my answer with a grain of salt, I am not expert in this subject,

If you do get a better answer, please let me know I am curious. (I would suggest to check as well with @AlienwareTech, they have been previously helpful to me

Regards

Thanks @euro, that is really useful summary and quite on point, as the implications for NVMe drives are pretty important. Would be easy to waste money for no gain.

Since the are only a finite number of PCIE lanes available, I assume that Alienware have had to make some hard decisions about allocating them. For example, some lanes will be allocated to the AGA even though for many people those lanes would be more useful for NVMe use. Also, my GTX970m shows as being 16x capable but running with only 8x lanes.