Thanks for your response.
The NUC model I am currently working on is NUC6i5SYH, however it effects other models as well.
The most common issue is booting from USB, Flash drives with Windows Install or Acronis, it also fails with USB DVD drives.
The NUCs don’t start out this way….they are working great, but all of a sudden, they stop. And the drive is listed in the boot list even if it has been removed.
One thing I have noticed is that when it stop booting from USB, it will list the boot type twice. (See attachment)
It isn't a UEFI/Legacy BIOS image issue. In this case, the USB drive was booting fine then suddenly stopped.
This message was posted on behalf of Intel Corporation
I apologize for any inconvenience.
Since you have several units, let try's focusing in one just for testing purposes.
Could you please let me know the BIOS version on the NUC6i5SYH?
Have you tried clearing the log event?
Check this post#10 on this thread Re: Nuc6i5syh F10 boot sequence non existent disk detected., the issue may be related, and if possible follow that suggestion.