https://support.microsoft.com/en-us...-update-blocked-because-of-drive-reassignment
"This PC can't be upgraded to Windows 10 May 2019" error on a computer that has a USB device or SD card attached
Cause
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.
Workaround
A USB device or SD card that's inserted into the computer can cause this issue to occur. To work around this issue, remove the external media and restart the May 2019 Update installation.
"This PC can't be upgraded to Windows 10 May 2019" error on a computer that has a USB device or SD card attached
Cause
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.
Workaround
A USB device or SD card that's inserted into the computer can cause this issue to occur. To work around this issue, remove the external media and restart the May 2019 Update installation.