Depends on which 7th gen NUC. I have a NUC7i7DNHE, and have an m.2 and 2.5" in it. You install the os on the m.2 and use the 2.5" as you want. You have to get the "tall" version of the nuic so you can have the 2.5" bay.
"W10 2004, W10 May 2020, W10 20H1, W10 19041.* - A rose, by any other name, still smells. "
- You need to install the both SSD and SO DIMM (see this video how to do this).
- Enter Bios - press F2 during boot when you see the Intel NUC logo.
- While in BIOS, press F9, followed by "Y" and then F10, followed by "Y", to save the default setting and exit from BIOS.
- Enter BIOS again and verify that both SSD are recognized: Advanced > Devices > SATA and Advanced > Devices > PCI
- Check the list of Compatible part for your NUC.