Microsoft confirms that the April 2026 patch update causes problems for many third-party backup tools on Windows 11
Facing problems with certain third-party apps after installing the April 2026 update on your Windows 11? Well, this issue is not due to your device but the Windows update released by Microsoft.
Recently, Microsoft has confirmed that after installing Windows updates released on or after 14th April, certain third-party backup applications that rely on the kernel driver psmounterex.sys might experience failures when attempting to mount or manage disk images. These Windows updates include routine security hardening to help protect devices by blocking third-party drivers with known vulnerabilities.
Users and IT administrators might observe the following behaviour after installing the update:
- Backup applications that rely on the kernel driver psmounterex.sys might fail to mount backup image files as virtual drives.
- Attempting to browse or restore from a backup image might result in errors or timeouts
- Failures might be followed by error messages, such as “The backup has failed because Microsoft VSS has timed out during the snapshot creation” or VSS_E_BAD_STATE.
- Event Viewer might show Code Integrity errors, including that psmounterex.sys was blocked from loading
- Backup creation (full image backups) may still succeed, but image mount operations will fail
The company has revealed that the psmounterex.sys driver will remain on the Microsoft vulnerable driver blocklist. Applications that depend on this driver will continue to experience failures until they are updated to a newer version that includes the required protections.
It is said that the affected programs include Acronis Cyber Protect, Macrium Reflect, NinjaOne Backup, UrBackup, and others that rely on this driver.
Microsoft has advised that if your backup software relies on the driver, you should update your backup software. Also, you can contact your backup software vendor for a compatible version that uses updated drivers. Uninstalling the patch is not advised, as it leaves your system vulnerable.
