I need to boot from a USB device, either flash drive or CD.

No matter what I do in the BIOS to select these alternate devices, win10 on the SATA drive always takes over and boots. I have tried every option in the bios, selected alternate targets using the function keys during boot, and googled every question I can think of.

I never had this problem with XP or win7. It appears that win10 just refuses to let boot happen from any other device somehow. I did find one similar question, where the answer was "you just need to disable Secure Boot and enable Legacy booting in the BIOS." Nothing in the BIOS that looked like that, but I tried flipping every option in the bios that was even remotely associated with booting.

I have the same problem on both of my win 10 machines, with different mobo manufacturers.

Any ideas?