Support is provided for using RAID across two M.2 NVMe SSDs or two SATA HDDs; you cannot use it if you have one M.2 NVMe SSD and one SATA HDD. Since the BN NUCs have one M.2 connector and one SATA connector, there are no "standard" configurations that can utilize RAID.
In that statement I very carefully qualified it using "standard". There are two non-standard (non-traditional) configurations that we can look at:
1. The M.2 connector does have support for using M.2 SATA SSDs. You could use an adapter that exposes the SATA lane to a standard SATA connector. Using this connector, you could connect a second HDD and then use RAID across the two SATA HDDs. Of course, this configuration is problematic as you must power both HDDs and there is no option for mounting two HDDs within the chassis.
2. Can you use RAID across one M.2 SATA SSD and one SATA HDD? In theory, yes, I think you can do this (at least for RAID1), but it would essentially be a waste of a SSD as all write operations will be at the speed of the slower HDD. It wouldn't surprise me if Intel blocked this combination for just that reason.
Clear as mud?
...S
