You could put it anywhere if you use some double sided tape. LOL.... but you might need some power cable extensions. I used a dell 2.5 inch drive adapter that fit into the HDD caddy and it holds two 2.5 inch drives in one bay. I ditched the HDD that came with it and used the adapter and I run two 2.5 SATA SSD in the front top bay and 2 NVMe drives on the motherboard. If you use both 3.5 bays for actual HDD's you will need to get creative on where and how to mount the SSD. There are 2 SATA power connectors in each drive bay but they are pretty close together, hence the reason for a SATA power extension cable. The 8950 comes with no extra data cables and to use the drive bay at the back you will need a pretty long cable.
My XPS 8950 is scheduled to be delivered on December 12th. It comes with one M.2 card and 1 3.5" SATA hard disk. I see that the motherboard has 3 SATA connectors in addition to one for the CD/DVD drive. I plan to add a second M.2 card and a second 3.5" hard disk.Now I'm looking at the third SATA port and thinking about adding a SATA SSD and wondering if anybody in this community has done this. If so, where did you put the SSD? Can you post a picture of it? Thanks.
XPS 8950, anybody install a 3rd SATA drive?