It can be difficult to work out how to get into BIOS on the NUC, especially if you have made some changes to UEFI settings or installed different (secure & unsecure) operating systems.
After countless Google searches which described different procedures for the various NUC models I found out that, for at least the dual-core Celeron model, the procedure for entering the Emergency Boot Mode.
- Power off the NUC.
- Press AND hold the power button for about 3 seconds to turn on the NUC.
- All going well you should arrive in a text-only maintenance menu.
There is an internal jumper on some models which will boot the NUC straight into the maintenance menu, however it can be a pain to have to open up the computer when you need to use it; especially if you are constantly trying out different operating systems.