I don't know if this is well placed here. But since some users might update early to W10 1903 here is a short info:
If you install the May 2019 Update ("1903") for Windows 10, you should pay a little attention to which devices are plugged into your computer. In a support article, Microsoft points out that external devices (Disks, USB-Sticks etc.) could prevent the update from being installed. It is currently distributed to developers via Microsoft's MSDN.
The reason for this behavior is the dynamic assignment of drive letters to such external disks. This applies to users who use SD cards or USB devices such as CD drives or sticks.
Microsoft wants to prevent this with the error message. Unfortunately, the error message does not give any details about why the error occurred. There is only a generic message that the computer is supposedly not ready for an update.
Microsoft plans to solve the problem, but initially only for insiders. It only affects users who use the October-2018 or April-2018 update as the basis for updating to the May-2019 version.
1903 was released May 21 2019 (yesterday).
I, amongst others, believe that the usb/sd restriction is a microsoft plot to stop users with devices that have limited storage from using a usb or sd to supplement their storage, especially for an install. The dastardly plot kills a lot of low storage devices from being able to upgrade. Worse yet, you cannot use a usb stick for an UPGRADE because of this restriction.
Doc (not an intel employee or contractor)
I've just upgraded my NUC8i7HVK to version 1903. It did forced upgrade, using Microsoft Media Creation Tool. While the Windows image was downloaded from Microsoft , 3GB WD Elements external HDD was connected to my NUC. The external HDD was connected also throughout all upgrade process. The upgrade to version 1903 with this configuration, ended successfully. I'll will try the same upgrade on NUC7i7DNHE, but this time from external USB stick.