Microsoft has published a list of known Windows 10 issues (version 1903), which will block the installation of Windows Update 10 May 2019 Update on affected computers.
Microsoft plans to continue testing Windows 10 (version 1903) until the end of May, and only after this update (possibly) will become publicly available. Why is it possible? Microsoft has published a list of issues that will block the installation of updates on affected devices.
List of Known Issues That Block Windows Installation 10 May 2019 Update
External drives are connected. If external storage media, a USB drive or an SD memory card are connected to your computer, then you will be blocked from updating to Windows 10 May 2019 Update. If you disconnect all external USB devices and / or SD memory cards, the update should be installed normally. This problem will be solved in a future update for Windows 10. For insiders, this problem is solved in Windows 10 build 18885 (20H1) and later builds.
Old versions of anti-cheat programs for games. If older versions of the anti-cheat program used with games are installed on your computer, you will be blocked from Windows 10 May 2019 Update. Some older versions of anti-cheat programs can cause crashes (GSOD) on a PC after upgrading to Windows 10 (version 1903). Microsoft is working with partners to upgrade their software, and most game developers have released fixes to prevent this problem from occurring. To minimize the chance of blocking, make sure that you are using the latest versions of your games before attempting to update the operating system.
Redirect known folders. If you redirect any known system folders (for example, Desktop, Documents, Images), an empty folder with the same name will be created (hi, Windows 10 October 2018 Update) in your% userprofile% directory. !) Known folders are redirected if you decide to enable the creation of backup copies of your files in OneDrive using the OneDrive Setup Wizard, or if you decide to keep backup copies of your content in the settings when you first start Windows on your computer. You may also have redirected known folders manually through the Properties menu in Explorer. This problem does not delete any user files, and a fix is being prepared.
Recall that Microsoft has made changes to the policy for obtaining new updates of the function – the installation of the new version will not be forced if users use supported versions of Windows 10.