XPS 8960, confused about storage configuration

You can install 2 x 2.5" SSD instead of 2 x 3.5" HDD. But you may need a conversion kit to install the 2.5" drives instead of the 3.5" drives. This statement on the spec sheet explains it: NOTE: One 3.5-inch hard drive can be changed to two 2.5-inch hard drives by purchasing the conversion kit from Dell sales agent.

Yes, understand that part. I already have a place in my case for two 2.5 drives. At least that's what the documentation says. What I am needing to know is if the motherboard itself will allow me to use the 3 extra drives. I only ask because I have 3 sata ports and an extra M.2 on the new dell desktop, however that doesn't mean the MB will see all of them. Like with my old desktop, the MB has 2 M.2 slots and 6 sata ports but if you use the M.2 slots on the MB then you can only use 4 drives that plug into the sata ports if that makes sense. My question is does this motherboard also see the M.2 the way my old one did. If so then I'm down a sata port already with the M.2 the OS is on. Which means I can only use two of the 3 drives and I will need to turn one into an external. I was just trying to sort this out before I started opening the case and poking around so I can decide which drives I want "on board" and which one is going to be the external one. Or I could always buy an expansion card. I keep forgetting those exist.

If the old PC used SATA M.2 SSDs, that might explain things.

XPS 8960 only uses NVME M.2 SSDs, and NVME SSDs don't use SATA data lines..

If necessary, you could always image one of the 2.5" SATA drives onto a new NVME SSD in the 2nd (empty) M.2 slot, which would eliminate that SATA drive from your setup. 

You can install the OS in RAID mode using M.2 2230 or M.2 2280 PCIe NVMe solid-state drives, and then install both 3.5-inch hard drives.