XPS 17 9720 Setup and Specifications gives details. It says:"This system supports two solid-state drives. Each slot supports a M.2 2230 or M.2 3380 solid-state drive." I think "3380" is a typo, as it more properly lists "2280" elsewhere on that page. Crucial shows some compatible drives. You can compare to the specs of the one you're interested in.   
Hello, I'd like to install second 4Tb SSD hard drive into my XPS 9720. What models are compatible? As far as I understand it should have PCle Gen4.0 connection and it should be thin. For example, is Kingston Fury Renegade PCle 4.0 NVMe M.2 SSD compatible? Thanks. Sergey.
Second SSD for XPS 9720