You could not run an OS off the M.2 mounted on an adapter. The 790 does not have an M.2 slot on the MB, and M.2 SSD are NVMe/PCIe drives. The 2 choices in SSD are either SATA or NVMe/PCIe. This forum post has a huge amount of information about M.2 drives and the 790.
Please may I enquire as a novice. I am helping a friend get his PC up and running. We have a PCI card on to which one can mount an SSD m.2 card.  I know very little about these cards, but have read a couple of articles. Please does anyone know whether an m.2 card fitted to the pci adapter  will work. Does it matter if it is NVME or PCI-E or otherwise. Could one run the OS off this drive On the other hand is there a slot on the motherboard to mount one (2280 size maybe?) Would I need to make any changes to the bios It is somewhat bewildering to have to make these considerations. On the other hand with a simple SATA drive one just knows one has to merely plug it in and it is done with. May I thank any forthcoming help. Ian
Request advice Optiplex 790 - m.2 drive