Microsoft says it partially resolved issue which would cause Windows 10 version 1903 upgrades being blocked by improper drive reassignment during installation when external USB device or memory cards are attached. Windows 10 May 2019 Update will be automatically blocked if external media is attached with an error message saying “This PC can’t be upgraded to Windows 10″” Microsoft is currently working on a full fix for the issue which may block Windows 10 upgrades if USB drives or SD cards are plugged into the computer
Source: security