This blog entry is about how I solved the problem where I was no longer able to enter the BIOS using F2 on my Intel NUC D54250WYK.
The problem started when I upgraded my BIOS from 22 to 25 and enabled SecureBoot. Once I did that, the NUC would boot directly into Windows without showing the NUC boot logo nor the F2/F7 prompt.
This is not an uncommon problem. In my case, it took a combination of solutions to solve the problem.
First, the fact that the NUC logo wasn't showing was a clue that the NUC BIOS wasn't detecting the monitor properly. Apparently there's some sort of detection for the monitor to support 1024x768. This detection seems flaky. I tried the DisplayPort connector on two separate monitors without success.
The solution for this problem was to use an Apple mini-DisplayPort to DVI adapter to go from mini-DisplayPort on the NUC to DVI on the monitor. Once I connected the monitor in this manner, the boot screen showed up immediately. I got the idea from this article, which recommended mini-DisplayPort to HDMI.
The 1024x768 detection is particularly problematic if you are connecting the NUC to a TV or through a receiver. In these cases, you may need to connect to a "real" computer monitor.
The next problem was the SecureBoot prevented the BIOS from showing. To solve this it was necessary to enter Maintenance Mode, as described at https://communities.intel.com/message/190524.