[SOLVED] SnakeOil OS suddenly refuses to boot. Help needed!

[agent_kith]

Hello! 
I have installed SnakeOil OS on an old 5i5 Intel NUC's SSD, everything went smoothly and it sang beautifully. The sound quality was amazing!

Alas, yesterday I have decided to upgrade the NUC's BIOS to the latest version (it is a long story why) and I have discovered that the SnakeOil OS suddenly would not boot any more! 
In BIOS, in both UEFi and Legacy boot sections, there are no bootable drives listed. Entering F10 boot selection shows no bootable drives.
I have tried all possible BIOS settings (secure boot is disabled, etc.), but no luck. 
I have decided to reinstall it, booted the install iso from USB flash drive, went through the whole install procedure, everything seemed fine, worked exactly as the first install, but upon reboot I'm still getting the message: "reboot and select proper boot device or insert boot media in selected boot device".
Any ideas?

PS
This is my NUC model, it has a 120GB SSD and 8GB of RAM:
https://www.notebookcheck.net/Intel-NUC-...978.0.html

Whats your SATA device settings? Set to AHCI or something else (e.g. some sort of RAID).

Try and reset your BIOS to the factory default and see how that goes. Once you can get your SSD detected as a boot device, I would think it'll work. Any settings in the CSM you think is the issue?