Microsoft won't be rolling out the Windows 10 May 2019 Update (1903) until next month, though it went live on the Microsoft Developer Network late last week shortly after its SDK became generally available. However, many testers have experienced a nagging issue with installing the update on their machines.
In a support document published today, Microsoft says PCs with either the Windows 10 April 2018 Update (version 1803) or the October 2018 Update (version 1809) installed are being blocked from upgrading to the latest version if an external USB device or SD card is attached to them. Affected users are receiving the following error message when attempting to install 1903:
Microsoft blames the issue on what it describes as "inappropriate drive reassignment" which may take place in machines with a USB device or SD card inserted during installation. As you may be aware of already, Windows assigns a letter to every external device attached to a PC. Issues may arise when, after software installation, a certain external drive is assigned a letter that's different from the one given to it before the upgrade. Besides external storage devices, the drive reassignment may also affect internal hard drives.
Fortunately, early adopters can work around the issue by removing any external storage devices attached to their PCs and then restarting the update installation. It should be mentioned that the problem does not affect PCs running older versions of Windows 10.
It's not at all irregular for the May 2019 Update to run into this type of problem since it's still in the final phase of testing, but here's hoping that Microsoft will fix the issue once the update becomes available to the public in May.