Just downloaded KB4103721. After the download a restart was required, I did that. When the boot had completed, the screen went Black and a small rotating circle appeared. That stayed on the screen for 15 minutes. I finally gave up and forced a shut down.

Restarting, the boot sequence again completed and the Black screen again appeared. Another 15 minute wait and a shut down. Trying to figure out how I was going to change the boot sequence to the USB port, in frustration I installed a Win 10 iso USB stick. Another restart and it finally brought the complete boot and desktop to the screen. The USB stick had nothing to do with it starting to work. No idea why it took so long to install this update but just thought I'd post my experience so others may be aware of this "feature"

I did the Spring Creators update last week so this PC (Acer Aspire One) was as up to date as far as MS concerns.
Regards,