A week ago, I posted that I could not upgrade my Windows 10 PCs to the latest version, v1903, May 2019. What I didn’t do was take the time to research the error. Today I did.
The problem is the latest version will change the drive letter on your USB devices. I was trying to install using a USB thumb drive.
The solution is to copy the entire thumb drive to your local hard drive or copy it to your network shared drive. Then run the setup program. That works.
I would suggest allocating an hour for the upgrade.