If the device is connected to an external USB device or card reader, the computer will prevent the Windows 10 May 2019 (Version 1903) feature update from being upgraded. In a technical consultation released on April 24, Microsoft explained that when an external USB device or SD card reader is connected, it will cause a new version of Windows 10 to fail.
Inappropriate drive reassignment can occur on eligible computers that have an external USB device or SD memory card attached during the installation of the May 2019 update. For this reason, these computers are currently blocked from receiving the May 2019 Update. This generates the error message that is mentioned in the “Symptoms” section if the upgrade is tried again on an affected computer.
Example: An upgrade to the May 2019 Update is tried on a computer that has the October 2018 update installed and also has a thumb drive inserted into a USB port. Before the upgrade, the device would have been mounted in the system as drive G based on the existing drive configuration. However, after the upgrade, the device is reassigned a different drive letter. For example, the drive is reassigned as drive H.
The easiest way to solve this problem is to remove the external USB device or SD card until the upgrade of Windows 10 May 2019 is complete.