Microsoft will block upgrades to the Windows 10 May 2019 Update if external media such as a USB device or SD card is attached to the computer. This is being done as USB devices can have their drive letters reassigned during the upgrade. Microsoft has also stated that this issue will be fixed in a future Windows 10 servicing update. Users will first need to remove any external media and then reboot the computer to try the May 2019 update again. The issue is resolved in build 18877 and later Windows 10.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-may-2019-update-to-be-blocked-if-using-usb-drives/