Did you update the bios? If not, try updating it to latest bios version.
In the case you updated the bios and issue started, try with a bios recovery. Here you have some instructions to do so:
The NUC was updated to 350 (latest BIOS) when it happened.
In the end, I went back to my dealer and they exchange it for a new NUC.
The same SSD is now showing in the BIOS of the new unit and all is fine (albeit, after only 2 days). Hope the problem is not going to surface again.