Well, this was a little vexing…
My go-to computer on the road is a MacBook Pro (Retina), which I use Apple’s Boot Camp software on to allow it to simultaneously be a bad-ass MacOS X computer and a killer Windows machine. It really is a terrific laptop.
While on a trip this week, I got a notice (on the Windows side) that Windows 10 was ready to download as a free update to the Windows 7 I had installed. Having done a flawless upgrade of my main desktop to Windows 10, I unhesitatingly clicked “Upgrade”, and an hour or so later, I was staring at the Windows 10 login screen on my newly upgraded laptop. “Press Ctrl+Alt+Delete to Login” it instructed…
I pressed it. Nothing happened. Pressed it again.. three times.. a dozen times… nothing. Rebooted. Powered the machine off. Let it wait for a while. Switched to the Mac side. Switched back. (At least I knew that the keyboard was sorta-working, as it recognized the Alt/Option key for switching boot partitions). But everytime I got to the Windows login screen, I was SOL.
After my trip, I plugged in an external keyboard and it instantly got past the login screen. I installed updates to Windows 10 but when I rebooted, the same problem persisted. What the devil was going on?
Turns out, the Apple keyboard drivers that came with Boot Camp got deactivated or otherwise messed with by the upgrade, so it was unable to handle the delete key in the upper-right of the built-in keyboard (most likely treating it as backspace instead).
The workaround: To hit Command-U at the login to activate the accessibility settings and use the onscreen virtual keyboard to trigger a Ctrl-Alt-Delete.
The fix: Boot back over to the Mac side, go to the Utilities folder and launch Boot Camp assistant. Download the (sorta) latest drivers.* onto a USB thumb drive. Then boot back over to Windows, pull the virtual keyboard trick to log in, then run the setup program on the flash drive to repair the drivers. Reboot, and all is well.
Hope this helps someone!
*It actually installs drivers from 2014, but attempting to directly download the Boot Camp 5.1xxx drivers from the Apple site results in an installer that refuses to run on this machine–even though it was specifically listed among the supported machines on the Apple site).