There’s much to be said for keeping up to date with new releases, and Microsoft’s 1903 major update should be implemented as soon as possible. However, the update is not necessarily straightforward, as I discovered.
Towards the end of a lengthy process I received an error message to say that the installation had failed. Looking further into it, I discovered it was because I had a USB drive connected. I disconnected it and started the install again (which includes the download). It did pick up where I left off, however, and then it started the drawn-out process of implementing the changes.
Microsoft is aware of the problem. Nevertheless, it would have helped if it had warned me ahead of time.
15 July 2019: The latest release has now fixed this problem.