Windows April Updates Trigger Backup Application Failures

Microsoft has confirmed that the security updates released on April 2026 for Windows are causing serious failures in third‑party backup applications that rely on the psmounterex.sys driver. The issue, which first surfaced on community forums and support tickets, manifests as repeated error messages and the inability to complete backup jobs after the patches are installed.

The psmounterex.sys driver, used by many backup solutions to interface with Windows storage APIs, appears to be incompatible with changes introduced in the April cumulative updates. Microsoft’s release notes highlight modifications to the driver signing requirements and the handling of I/O request packets, which inadvertently break the driver’s ability to map volumes correctly. As a result, backup software that depends on this component can no longer enumerate or protect disks, leading to incomplete or failed backups.

Organizations that rely on these backup tools for data protection and disaster recovery are advised to monitor their backup logs for the specific error code associated with psmounterex.sys. Microsoft recommends applying the latest out‑of‑band hotfix, which reverts the driver signing policy for affected builds while a permanent fix is developed. In the meantime, users can temporarily disable the problematic driver, switch to a backup solution that does not use psmounterex.sys, or roll back the April updates until a patched version is available.

The incident underscores the broader challenge of maintaining compatibility between Windows updates and ecosystem drivers. Microsoft has opened a dedicated support channel for affected vendors and is working with them to validate updated drivers ahead of future rollouts. Security teams should treat this as a reminder to test critical updates in a staged environment and maintain incident response procedures that account for third‑party component failures.