In the meantime I've solved the issue.
It is very strange, I can't really explain. I switched off the SATA part in the BIOS, booted into Fedora. After that, again in BIOS, I switched SATA on again, left the BIOS, hit F10 for boot menu and voila: both drives were listed there.
This message was posted on behalf of Intel Corporation
I am glad to hear that you were able to track this down. The only thing I would mention is to take into consideration the boot priority, make sure you select the drive that contains the operating system in order for the Intel® NUC Kit to boot properly.
If you have any question, please let me know.