I install every patch that comes along so assumed it was up-to-date
Bryan is 100% correct, you have a old Build, that's actually build 1803, which is close to a year out of date.

If you open Winver Via the search it should show, prob 1803 ( highlighted in the picture).I've manually upgraded to the latest:

Click image for larger version. 

Name:	winver_Latest.png 
Views:	23 
Size:	19.1 KB 
ID:	9550

What more than likely has happened in when 1809 came out ( the current public build) it was pulled /stopped because of a bug, when MS re -released it again, a HUGE number of computers didn't upgrade.

Having the second drive as D is not a good idea.

Cant explain the technical details right now ( SWMBO) is calling.

BUT as a quick experiment, go into drive manager, re assign a different Drive letter to the current D, F,H I -- what ever anything but D.