Cumulative Update KB4493440 Win 10 v1709 Build 16299.1127 April 25 Win Update

I have always assumed it is on the drive but since you ask the question, the only answer I have is - I don't know.
Where is it normally installed?
I do have a second internal hard drive for storing my back ups. Could having two internal hard drives be causing the issue?
Would some screen shots of my current configuration help?

Sorry I've not tried to attach images here before and it looks like I got one or two that are not needed and I don't see how to remove them.

No not all. Just my external back up hard drive. Printer, scanner(s), wireless keyboard, external storage, etc. were still connected. Previously I had tried with all of these disconnected. Same result at that time. Will try again if that is your reco. That may take a few days.
Again, where does Windows install if not on the hard drive?

Upgrading with the flash drive (setup.exe) works if you disconnect Internet (wired & wireless). Disconnecting the external HDD would be safer. Otherwise, if you still have the ISO image file, setup.exe will work without any special requirements. But, removing USB devices is safer.

If still not working, a screenshot of Minitool Partition Wizard would be helpful. Get the free trial there and install it with no Internet so that you don't get any offers, such as Google chrome or Avast.

Best Free Partition Manager for Windows | MiniTool Partition Wizard Free

No, Just trying to get the latest build installed so I can continue to receive quality and security updates while keeping my programs and files.

yes