So, this is the real reason why you can not boot from USB (see your thread Can't boot from USB ). By, the way. How did you disable SATA in bios if you can not enter it?
Try the following procedure to enter Bios setting:
1. Connect speaker or headphone to the audio port on NUC.
2. Depress Power Button and hold it for about 3 seconds.
3. Release Power Button after hearing 3-td beep (after about 3 seconds).
4. Wait till you will see Power Button Menu.
5. Press F2 to enter Bios setting.
6. Press F9 to bring back Bios defaults and than F10 to save the default settings.
Note: If you did not succeed to enter the Power Boot Menu and instead, NUC boot to your Libreelec , means that you have hold Power Button to short or to long. Try again and release after third beep.
Now, if setting the Bios to default, did not help, you shall try to recover your Bios (from the same Power Boot Menu, by pressing F4 or using bios secure jumper method): BIOS Recovery Update Instructions for Intel® NUC
In order to further help you, could you please confirm the following information:
-A full description of your issue?
-The model of you NUC?
-The Operating System that you are using?
Hope I can hear from you soon.