Hello cbwhsl ,
In M.2 slot you can install SSD with NVMe bus interface or with SATA bus interface. In 2.5" bay you can install HDD or SSD only with SATA interface. The performance of SSD with NVMe interface is much better than SSD with SATA interface. However the SSD with NVMe interface is still quite expensive. So, in my NUC I have installed 250GB SSD with NVMe interface in the M.2 slot. This is my PROGRAM disk. On this SSD I've installed Windows and all other programs I use (for example Office, Photoshop, Pinnacle Studio and others). Still I got more that 170GB free on this disk. In the 2.5" bay I've installed 1TB HDD (which is very chip). This is my DATA disk. On this HDD I keep all my data (i.e. documents, pictures, video files, downloads,...). In such configuration I've high performance PC with cheap storage for data.
Of course, if you prefer to use only one SSD/HDD, you can do this. It will work installed in M.2 slot or in 2.5" bay.
Hope this helps
Leon
