I'm dual booting Windows 10 on one SSD and Windows 7 on the other SSD; motherboard is the ASUS Sabertooth Z170. Secure Boot suddenly stopped working for Windows 7 but not for Windows 10. Today, on the Windows 7 install, a few updates came through on Windows Update: KB3138901, KB3139923, KB3137061, KB3133977, none of which seem to have anything particularly to do with Secure Boot. After installing them, I rebooted, and it booted up fine. I then updated Steelseries Engine (for my Steelseries Rival mouse) from 3.6.6 to 3.6.7, then rebooted, and the red alert window came up talking about a Secure Boot violation, unauthorized changes, etc. Booting up in Safe Mode was not possible, and the Secure Boot setting in the BIOS was greyed out and not changeable in a straightforward manner. I eventually learned how to disable Secure Boot on current motherboards by backing up then deleting the PK Secure Boot Key. Now that I could boot into Windows 7 again, I uninstalled Steelseries Engine 3.6.7 and installed 3.6.6, then turned Secure Boot back on by restoring the PK key that I had backed up, but Secure Boot still didn't work. I then uninstalled the four KBs and Secure Boot still doesn't work. I guess I just won't use Secure Boot again until I'm ready to re-install Win7 on that SSD, which I won't do until after the only reason I'm using it now (to run Skyrim, because DirectX9 has a 4GB limit for VRAM usage in Windows 8 and later). I'm baffled as to what happened to make Secure Boot go nuts, though, and what I could possibly do to find out what Secure Boot has a problem with. As I read through various Google searches, I get the impression that the question is more like "why was Secure Boot working with Windows 7 at all?" To be clear on one point: through all this, the Windows 10 install had no issues and worked fine with Secure Boot on.