Microsoft has removed the safeguard hold on PCs with an external USB device or SD memory card that prevented them from upgrading to Windows 10 May 2019 update. These PCs should now be able to upgrade to the May 2019 update, as Microsoft asked to wait for up to 48 hours per the update provided on July 11.
You can read about all the top features that the May 2019 update aka Version 1903 brings in detail by going to our May 2019 update dedicated page.
Microsoft describes the issue as,
If you have an external USB device or SD memory card attached when installing Windows 10, version 1903, you may get an error message stating “This PC can’t be upgraded to Windows 10.” This is caused by inappropriate drive reassignment during installation.
Sample scenario: An update to Windows 10, version 1903 is attempted on a computer that has a thumb drive inserted into its USB port. Before the update, the thumb drive is mounted in the system as drive G based on the existing drive configuration. After the feature update is installed; however, the device is reassigned a different drive letter (e.g., drive H).