Microsoft is currently investigating an inappropriate drive reassignment bug that prevents Windows 10 1903 or the May 2019 update Setup from proceeding and as such, is not offering the update to devices that have USB storage devices plugged in. The issue will be resolved in a future servicing update and in the meanwhile, a workaround is to remove all external storage devices before initiating the update procedure.
https://www.notebookcheck.net/Windows-10-May-2019-update-currently-being-blocked-on-computers-with-attached-USB-or-SD-Card-storage.418772.0.html