A:Answer From Dell website Q&A for the New Inspiron 7791 7000 17 inch 2-in-1 laptop
This model supports One 2.5-inch Hard Drive and One M.2 2230/2280 Solid State Drive.
NOTE: The solid-state drive is the primary drive.
The storage specifications designed and tested to work are :
→ M.2 2230 solid-state drive, PCIe NVMe 3 x4, Up to 1 TB
→ M.2 2280 solid-state drive, PCIe NVMe 3 x4, Up to 1 TB
→ One 2.5-inch hard drive, SATA AHCI 6 Gbps, Up to 2 TB
MY COMMENTS
There are 2 slots - one PCIe MVNe slot and one 2.5-inch SATA HD slot. Currently Dell sells only configuations with a single PCI3 NVMe 256 GB drive or a SATA 2.5-inch 256GB drive with a Intel Optane 32BG PCI3 accelerator (in the PCEe slot), but these can be upgraded - either by Geek Squad or you can do it yourself if you don't mind taking the bottom cover off and removing a couple things (Dell has Service Manual on web site available for this computer documenting how to remove/replace everything).